Merge branch 'dev' into chore/docker

2025-03-13 21:11:28 +01:00 · 2025-03-13 21:11:28 +01:00 · 441bf990e7
commit 441bf990e7
parent a2be18d816 6f194d9fab
149 changed files with 5420 additions and 780 deletions
--- a/.github/ISSUE_TEMPLATE/bug-report.md
+++ b/.github/ISSUE_TEMPLATE/bug-report.md
@ -24,6 +24,14 @@ Een duidelijke, beknopte beschrijving van wat je verwacht dat er gebeurt.
 Indien van toepassing, voeg een screenshot toe die het probleem duidelijk maakt.

 **Extra context**
-Voeg extra context over het probleem toe. Was je ergens bijzonder mee bezig of naar op zoek?
+Was je ergens bijzonder mee bezig of naar op zoek? Welke documentatie of links heb je (al) geraadpleegd?

- [ ] Ik heb aan deze issue het juiste label toegekend, afhankelijk van frontend, backend, ...
+<!--
+## Richtlijnen
+
+Zorg ervoor dat het volgende in orde is voordat je de issue aanmaakt:
+
+- Het is duidelijk waar de bug vandaan komt.
+- Ik heb aan deze issue het juiste label toegekend, afhankelijk van frontend, backend, ...
+- Ik heb de issue toegekend aan de juiste milestone.
+-->
--- a/.github/ISSUE_TEMPLATE/feature-request.md
+++ b/.github/ISSUE_TEMPLATE/feature-request.md
@ -6,7 +6,7 @@ labels: enhancement
 assignees: ''
 ---

-**Is your feature request related to a problem? Please describe.**
+**Is jouw feature request gerelateerd tot een probleem? Beschrijf.**
 Een duidelijke, beknopte beschrijving van het probleem. Wat mist er? Wat kan beter?

 **Beschrijf de oplossing die je zou willen**
@ -15,4 +15,11 @@ Een duidelijke, beknopte beschrijving van wat je zou willen dat er gebeurt.
 **Extra context**
 Extra context of screenshots bij de feature.

- [ ] Ik heb aan deze issue het juiste label toegekend, afhankelijk van frontend, backend, ...
+<!--
+## Richtlijnen
+
+Zorg ervoor dat het volgende in orde is voordat je de issue aanmaakt:
+
+- Ik heb aan deze issue het juiste label toegekend, afhankelijk van frontend, backend, ...
+- Ik heb de issue toegekend aan de juiste milestone.
+-->
--- a/README.md
+++ b/README.md
@ -9,29 +9,26 @@ Figma</a></span>
 Projectopgave</a></span>
 </p>

-<ul align="center" style="list-style-type: none">
-<li>Projectleider: Fransisco Gabriel Van Langenhove (<a href="https://github.com/Gabriellvl">@Gabriellvl</a>)</li>
-<li>Technische lead: Tibo De Peuter (<a href="https://github.com/tdpeuter">@tdpeuter</a>)</li>
-<li>Systeembeheerder: Timo De Meyst (<a href="https://github.com/kloep1">@kloep1</a>)</li>
-<li>Customer relations officer: Adriaan Jacquet (<a href="https://github.com/WhisperinCheetah">@WhisperinCheetah</a>)</li>
-</ul>
-
 Dit is de monorepo voor [Dwengo-1](https://sel2-1.ugent.be), een interactief leerplatform waar leerkrachten opdrachten
 en lessen kunnen samenstellen hun leerlingen en hun vooruitgang kunnen opvolgen.

 ## Installatie

-Om de applicatie in te stellen voor een productieomgeving, volg de [installatiehandleiding](https://github.com/SELab-2/Dwengo-1/wiki/Administrator:-Productie-omgeving).
+Om de applicatie in te stellen voor een productieomgeving, volg
+de [installatiehandleiding](https://github.com/SELab-2/Dwengo-1/wiki/Administrator:-Productie-omgeving).

 Alternatief kan je één van de volgende methodes gebruiken om de applicatie lokaal te draaien.

 ### Quick start

-1. Installeer Docker en Docker Compose op je systeem (zie [Docker](https://docs.docker.com/get-docker/) en [Docker Compose](https://docs.docker.com/compose/)).
+1. Installeer Docker en Docker Compose op je systeem (zie [Docker](https://docs.docker.com/get-docker/)
+   en [Docker Compose](https://docs.docker.com/compose/)).
 2. Clone deze repository.
-3. In de backend, kopieer `.env.example` (of `.env.development.example`) naar `.env` en pas de variabelen aan waar nodig.
+3. In de backend, kopieer `.env.example` (of `.env.development.example`) naar `.env` en pas de variabelen aan waar
+   nodig.
 4. Voer `docker compose up` uit in de root van de repository.
-5. Optioneel: Configureer de applicatie aan de hand van de [configuratiehandleiding](https://github.com/SELab-2/Dwengo-1/wiki/Administrator:-Productie-omgeving#dwengo-1-configuratie).
+5. Optioneel: Configureer de applicatie aan de hand van
+   de [configuratiehandleiding](https://github.com/SELab-2/Dwengo-1/wiki/Administrator:-Productie-omgeving#dwengo-1-configuratie).

 ```bash
 docker compose version
@ -47,7 +44,8 @@ docker compose up

 ### Handmatige installatie

-Zie de submappen voor de installatie-instructies van de [frontend](./frontend/README.md) en [backend](./backend/README.md).
+Zie de submappen voor de installatie-instructies van de [frontend](./frontend/README.md)
+en [backend](./backend/README.md).

 ## Architectuur

@ -59,8 +57,31 @@ De tech-stack bestaat uit:
 - **Backend**: TypeScript + Node.js + Express.js + TypeORM + PostgreSQL
 - **Identity provider**: Keycloak

-Voor meer informatie over de keuze van deze tech-stack, zie [designkeuzes](https://github.com/SELab-2/Dwengo-1/wiki/Developer:-Design-keuzes).
+Voor meer informatie over de keuze van deze tech-stack,
+zie [designkeuzes](https://github.com/SELab-2/Dwengo-1/wiki/Developer:-Design-keuzes).
+
+## Testen
+
+Voer volgende commando's uit om de <frontend/backend> te testen:
+
+```
+npm run test:unit
+```

 ## Bijdragen aan Dwengo-1

 Zie [CONTRIBUTING.md](./CONTRIBUTING.md) voor meer informatie over hoe je kan bijdragen aan Dwengo-1.
+
+Deze rocksterren hebben bijgedragen aan Dwengo-1:
+
+| Naam                                                                                                                                                    | Functie                    |
+| ------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------- |
+| [<img src="https://github.com/WhisperinCheetah.png" width="100px"/><br/><sub><b>Adriaan Jacquet</b></sub>](https://github.com/WhisperinCheetah)         | Backend Lead               |
+| [<img src="https://github.com/Gabriellvl.png" width="100px"/><br/><sub><b>Francisco Gabriel Van Langenhove</b></sub>](https://github.com/Gabriellvl)    | Team Lead                  |
+| [<img src="https://github.com/geraldschmittinger.png" width="100px"/><br/><sub><b>Gerald Schmittinger</b></sub>](https://github.com/geraldschmittinger) | Database Administrator     |
+| [<img src="https://github.com/joyelle436.png" width="100px"/><br/><sub><b>Joyelle Ndagijimana</b></sub>](https://github.com/joyelle436)                 | Frontend Lead              |
+| [<img src="https://github.com/laurejablonski.png" width="100px"/><br><sub><b>Laure Jablonski</b></sub>](https://github.com/laurejablonski)              | Documentatie- en Test Lead |
+| [<img src="https://github.com/tdpeuter.png" width="100px"/><br/><sub><b>Tibo De Peuter</b></sub>](https://github.com/tdpeuter)                          | Technische Lead            |
+| [<img src="https://github.com/kloep1.png" width="100px"/><br/><sub><b>Timo De Meyst</b></sub>](https://github.com/kloep1)                               | System Administrator       |
+
+En in de toekomst misschien jij ook?
--- a/backend/README.md
+++ b/backend/README.md
@ -21,6 +21,14 @@ npm run build
 npm run start
 ```

+### Tests
+
+Voer volgend commando uit om de unit tests uit te voeren:
+
+```
+npm run test:unit
+```
+
 ## Keycloak configuratie

 Tijdens development is het voldoende om gebruik te maken van de keycloak configuratie die automatisch ingeladen wordt.
--- a/backend/eslint.config.ts
+++ b/backend/eslint.config.ts
@ -8,4 +8,14 @@ export default [
            globals: globals.node,
        },
    },
+
+    {
+        files: ['tests/**/*.ts'],
+        languageOptions: {
+            globals: globals.node,
+        },
+        rules: {
+            'no-console': 'off',
+        },
+    },
 ];
--- a/backend/package.json
+++ b/backend/package.json
@ -5,40 +5,46 @@
    "private": true,
    "type": "module",
    "scripts": {
-        "build": "NODE_ENV=production tsc --project tsconfig.json",
-        "dev": "NODE_ENV=development tsx watch --env-file=.env.development.local src/app.ts",
-        "start": "NODE_ENV=production node --env-file=.env dist/app.js",
+        "build": "cross-env NODE_ENV=production tsc --project tsconfig.json",
+        "dev": "cross-env NODE_ENV=development tsx watch --env-file=.env.development.local src/app.ts",
+        "start": "cross-env NODE_ENV=production node --env-file=.env dist/app.js",
        "format": "prettier --write src/",
        "format-check": "prettier --check src/",
        "lint": "eslint . --fix",
        "test:unit": "vitest"
    },
    "dependencies": {
-        "@mikro-orm/core": "6.4.6",
-        "@mikro-orm/postgresql": "6.4.6",
-        "@mikro-orm/reflection": "6.4.6",
-        "@mikro-orm/sqlite": "6.4.6",
-        "@types/js-yaml": "^4.0.9",
+        "@mikro-orm/core": "6.4.9",
+        "@mikro-orm/knex": "6.4.9",
+        "@mikro-orm/postgresql": "6.4.9",
+        "@mikro-orm/reflection": "6.4.9",
+        "@mikro-orm/sqlite": "6.4.9",
        "axios": "^1.8.2",
+        "cors": "^2.8.5",
+        "cross": "^1.0.0",
+        "cross-env": "^7.0.3",
        "dotenv": "^16.4.7",
        "express": "^5.0.1",
        "express-jwt": "^8.5.1",
-        "jwks-rsa": "^3.1.0",
-        "uuid": "^11.1.0",
+        "gift-pegjs": "^1.0.2",
+        "isomorphic-dompurify": "^2.22.0",
        "js-yaml": "^4.1.0",
+        "jsonpath-plus": "^10.3.0",
+        "jwks-rsa": "^3.1.0",
        "loki-logger-ts": "^1.0.2",
+        "marked": "^15.0.7",
        "response-time": "^2.3.3",
+        "uuid": "^11.1.0",
        "winston": "^3.17.0",
-        "winston-loki": "^6.1.3",
-        "cors": "^2.8.5",
-        "@types/cors": "^2.8.17"
+        "winston-loki": "^6.1.3"
    },
    "devDependencies": {
-        "@mikro-orm/cli": "6.4.6",
+        "@mikro-orm/cli": "6.4.9",
+        "@types/cors": "^2.8.17",
        "@types/express": "^5.0.0",
+        "@types/js-yaml": "^4.0.9",
        "@types/node": "^22.13.4",
        "@types/response-time": "^2.3.8",
-        "@types/js-yaml": "^4.0.9",
        "globals": "^15.15.0",
        "ts-node": "^10.9.2",
        "tsx": "^4.19.3",
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@ -2,8 +2,8 @@ import express, { Express, Response } from 'express';
 import { initORM } from './orm.js';

 import themeRoutes from './routes/themes.js';
-import learningPathRoutes from './routes/learningPaths.js';
-import learningObjectRoutes from './routes/learningObjects.js';
+import learningPathRoutes from './routes/learning-paths.js';
+import learningObjectRoutes from './routes/learning-objects.js';

 import studentRouter from './routes/student.js';
 import groupRouter from './routes/group.js';
--- a/backend/src/config.ts
+++ b/backend/src/config.ts
@ -1,10 +1,9 @@
-export const FALLBACK_LANG: string = 'nl';
+import { EnvVars, getEnvVar } from './util/envvars.js';

 // API
-
-export const DWENGO_API_BASE: string = 'https://dwengo.org/backend/api';
+export const DWENGO_API_BASE = getEnvVar(EnvVars.LearningContentRepoApiBaseUrl);
+export const FALLBACK_LANG = getEnvVar(EnvVars.FallbackLanguage);

 // Logging
-
 export const LOG_LEVEL: string = 'development' === process.env.NODE_ENV ? 'debug' : 'info';
 export const LOKI_HOST: string = process.env.LOKI_HOST || 'http://localhost:3102';
--- a/backend/src/controllers/learning-objects.ts
+++ b/backend/src/controllers/learning-objects.ts
@ -0,0 +1,69 @@
+import { Request, Response } from 'express';
+import { FALLBACK_LANG } from '../config.js';
+import { FilteredLearningObject, LearningObjectIdentifier, LearningPathIdentifier } from '../interfaces/learning-content.js';
+import learningObjectService from '../services/learning-objects/learning-object-service.js';
+import { EnvVars, getEnvVar } from '../util/envvars.js';
+import { Language } from '../entities/content/language.js';
+import { BadRequestException } from '../exceptions.js';
+import attachmentService from '../services/learning-objects/attachment-service.js';
+import { NotFoundError } from '@mikro-orm/core';
+
+function getLearningObjectIdentifierFromRequest(req: Request): LearningObjectIdentifier {
+    if (!req.params.hruid) {
+        throw new BadRequestException('HRUID is required.');
+    }
+    return {
+        hruid: req.params.hruid as string,
+        language: (req.query.language || getEnvVar(EnvVars.FallbackLanguage)) as Language,
+        version: parseInt(req.query.version as string),
+    };
+}
+
+function getLearningPathIdentifierFromRequest(req: Request): LearningPathIdentifier {
+    if (!req.query.hruid) {
+        throw new BadRequestException('HRUID is required.');
+    }
+    return {
+        hruid: req.params.hruid as string,
+        language: (req.query.language as Language) || FALLBACK_LANG,
+    };
+}
+
+export async function getAllLearningObjects(req: Request, res: Response): Promise<void> {
+    const learningPathId = getLearningPathIdentifierFromRequest(req);
+    const full = req.query.full;
+
+    let learningObjects: FilteredLearningObject[] | string[];
+    if (full) {
+        learningObjects = await learningObjectService.getLearningObjectsFromPath(learningPathId);
+    } else {
+        learningObjects = await learningObjectService.getLearningObjectIdsFromPath(learningPathId);
+    }
+
+    res.json(learningObjects);
+}
+
+export async function getLearningObject(req: Request, res: Response): Promise<void> {
+    const learningObjectId = getLearningObjectIdentifierFromRequest(req);
+
+    const learningObject = await learningObjectService.getLearningObjectById(learningObjectId);
+    res.json(learningObject);
+}
+
+export async function getLearningObjectHTML(req: Request, res: Response): Promise<void> {
+    const learningObjectId = getLearningObjectIdentifierFromRequest(req);
+
+    const learningObject = await learningObjectService.getLearningObjectHTML(learningObjectId);
+    res.send(learningObject);
+}
+
+export async function getAttachment(req: Request, res: Response): Promise<void> {
+    const learningObjectId = getLearningObjectIdentifierFromRequest(req);
+    const name = req.params.attachmentName;
+    const attachment = await attachmentService.getAttachment(learningObjectId, name);
+
+    if (!attachment) {
+        throw new NotFoundError(`Attachment ${name} not found`);
+    }
+    res.setHeader('Content-Type', attachment.mimeType).send(attachment.content);
+}
--- a/backend/src/controllers/learning-paths.ts
+++ b/backend/src/controllers/learning-paths.ts
@ -0,0 +1,64 @@
+import { Request, Response } from 'express';
+import { themes } from '../data/themes.js';
+import { FALLBACK_LANG } from '../config.js';
+import learningPathService from '../services/learning-paths/learning-path-service.js';
+import { BadRequestException, NotFoundException } from '../exceptions.js';
+import { Language } from '../entities/content/language.js';
+import {
+    PersonalizationTarget,
+    personalizedForGroup,
+    personalizedForStudent,
+} from '../services/learning-paths/learning-path-personalization-util.js';
+
+/**
+ * Fetch learning paths based on query parameters.
+ */
+export async function getLearningPaths(req: Request, res: Response): Promise<void> {
+    const hruids = req.query.hruid;
+    const themeKey = req.query.theme as string;
+    const searchQuery = req.query.search as string;
+    const language = (req.query.language as string) || FALLBACK_LANG;
+
+    const forStudent = req.query.forStudent as string;
+    const forGroupNo = req.query.forGroup as string;
+    const assignmentNo = req.query.assignmentNo as string;
+    const classId = req.query.classId as string;
+
+    let personalizationTarget: PersonalizationTarget | undefined;
+
+    if (forStudent) {
+        personalizationTarget = await personalizedForStudent(forStudent);
+    } else if (forGroupNo) {
+        if (!assignmentNo || !classId) {
+            throw new BadRequestException('If forGroupNo is specified, assignmentNo and classId must also be specified.');
+        }
+        personalizationTarget = await personalizedForGroup(classId, parseInt(assignmentNo), parseInt(forGroupNo));
+    }
+
+    let hruidList;
+
+    if (hruids) {
+        hruidList = Array.isArray(hruids) ? hruids.map(String) : [String(hruids)];
+    } else if (themeKey) {
+        const theme = themes.find((t) => t.title === themeKey);
+        if (theme) {
+            hruidList = theme.hruids;
+        } else {
+            throw new NotFoundException(`Theme "${themeKey}" not found.`);
+        }
+    } else if (searchQuery) {
+        const searchResults = await learningPathService.searchLearningPaths(searchQuery, language as Language, personalizationTarget);
+        res.json(searchResults);
+        return;
+    } else {
+        hruidList = themes.flatMap((theme) => theme.hruids);
+    }
+
+    const learningPaths = await learningPathService.fetchLearningPaths(
+        hruidList,
+        language as Language,
+        `HRUIDs: ${hruidList.join(', ')}`,
+        personalizationTarget
+    );
+    res.json(learningPaths.data);
+}
--- a/backend/src/controllers/learningObjects.ts
+++ b/backend/src/controllers/learningObjects.ts
@ -1,48 +0,0 @@
-import { Request, Response } from 'express';
-import { getLearningObjectById, getLearningObjectIdsFromPath, getLearningObjectsFromPath } from '../services/learningObjects.js';
-import { FALLBACK_LANG } from '../config.js';
-import { FilteredLearningObject } from '../interfaces/learningPath.js';
-import { getLogger } from '../logging/initalize.js';
-
-export async function getAllLearningObjects(req: Request, res: Response): Promise<void> {
-    try {
-        const hruid = req.query.hruid as string;
-        const full = req.query.full === 'true';
-        const language = (req.query.language as string) || FALLBACK_LANG;
-
-        if (!hruid) {
-            res.status(400).json({ error: 'HRUID query is required.' });
-            return;
-        }
-
-        let learningObjects: FilteredLearningObject[] | string[];
-        if (full) {
-            learningObjects = await getLearningObjectsFromPath(hruid, language);
-        } else {
-            learningObjects = await getLearningObjectIdsFromPath(hruid, language);
-        }
-
-        res.json(learningObjects);
-    } catch (error) {
-        getLogger().error('Error fetching learning objects:', error);
-        res.status(500).json({ error: 'Internal server error' });
-    }
-}
-
-export async function getLearningObject(req: Request, res: Response): Promise<void> {
-    try {
-        const { hruid } = req.params;
-        const language = (req.query.language as string) || FALLBACK_LANG;
-
-        if (!hruid) {
-            res.status(400).json({ error: 'HRUID parameter is required.' });
-            return;
-        }
-
-        const learningObject = await getLearningObjectById(hruid, language);
-        res.json(learningObject);
-    } catch (error) {
-        getLogger().error('Error fetching learning object:', error);
-        res.status(500).json({ error: 'Internal server error' });
-    }
-}
--- a/backend/src/controllers/learningPaths.ts
+++ b/backend/src/controllers/learningPaths.ts
@ -1,8 +1,9 @@
 import { Request, Response } from 'express';
 import { themes } from '../data/themes.js';
 import { FALLBACK_LANG } from '../config.js';
-import { fetchLearningPaths, searchLearningPaths } from '../services/learningPaths.js';
 import { getLogger } from '../logging/initalize.js';
+import learningPathService from '../services/learning-paths/learning-path-service.js';
+import { Language } from '../entities/content/language.js';
 /**
 * Fetch learning paths based on query parameters.
 */
@ -11,7 +12,7 @@ export async function getLearningPaths(req: Request, res: Response): Promise<voi
        const hruids = req.query.hruid;
        const themeKey = req.query.theme as string;
        const searchQuery = req.query.search as string;
-        const language = (req.query.language as string) || FALLBACK_LANG;
+        const language = (req.query.language as Language) || FALLBACK_LANG;

        let hruidList;

@ -28,14 +29,14 @@ export async function getLearningPaths(req: Request, res: Response): Promise<voi
                return;
            }
        } else if (searchQuery) {
-            const searchResults = await searchLearningPaths(searchQuery, language);
+            const searchResults = await learningPathService.searchLearningPaths(searchQuery, language);
            res.json(searchResults);
            return;
        } else {
            hruidList = themes.flatMap((theme) => theme.hruids);
        }

-        const learningPaths = await fetchLearningPaths(hruidList, language, `HRUIDs: ${hruidList.join(', ')}`);
+        const learningPaths = await learningPathService.fetchLearningPaths(hruidList, language, `HRUIDs: ${hruidList.join(', ')}`);
        res.json(learningPaths.data);
    } catch (error) {
        getLogger().error('❌ Unexpected error fetching learning paths:', error);
--- a/backend/src/data/content/attachment-repository.ts
+++ b/backend/src/data/content/attachment-repository.ts
@ -1,13 +1,37 @@
 import { DwengoEntityRepository } from '../dwengo-entity-repository.js';
 import { Attachment } from '../../entities/content/attachment.entity.js';
-import { LearningObject } from '../../entities/content/learning-object.entity.js';
+import { Language } from '../../entities/content/language';
+import { LearningObjectIdentifier } from '../../entities/content/learning-object-identifier';

 export class AttachmentRepository extends DwengoEntityRepository<Attachment> {
-    public findByLearningObjectAndNumber(learningObject: LearningObject, sequenceNumber: number) {
+    public findByLearningObjectIdAndName(learningObjectId: LearningObjectIdentifier, name: string): Promise<Attachment | null> {
        return this.findOne({
-            learningObject: learningObject,
-            sequenceNumber: sequenceNumber,
+            learningObject: {
+                hruid: learningObjectId.hruid,
+                language: learningObjectId.language,
+                version: learningObjectId.version,
+            },
+            name: name,
        });
    }
+
+    public findByMostRecentVersionOfLearningObjectAndName(hruid: string, language: Language, attachmentName: string): Promise<Attachment | null> {
+        return this.findOne(
+            {
+                learningObject: {
+                    hruid: hruid,
+                    language: language,
+                },
+                name: attachmentName,
+            },
+            {
+                orderBy: {
+                    learningObject: {
+                        version: 'DESC',
+                    },
+                },
+            }
+        );
+    }
    // This repository is read-only for now since creating own learning object is an extension feature.
 }
--- a/backend/src/data/content/learning-object-repository.ts
+++ b/backend/src/data/content/learning-object-repository.ts
@ -1,14 +1,34 @@
 import { DwengoEntityRepository } from '../dwengo-entity-repository.js';
 import { LearningObject } from '../../entities/content/learning-object.entity.js';
 import { LearningObjectIdentifier } from '../../entities/content/learning-object-identifier.js';
+import { Language } from '../../entities/content/language';

 export class LearningObjectRepository extends DwengoEntityRepository<LearningObject> {
    public findByIdentifier(identifier: LearningObjectIdentifier): Promise<LearningObject | null> {
-        return this.findOne({
-            hruid: identifier.hruid,
-            language: identifier.language,
-            version: identifier.version,
-        });
+        return this.findOne(
+            {
+                hruid: identifier.hruid,
+                language: identifier.language,
+                version: identifier.version,
+            },
+            {
+                populate: ['keywords'],
+            }
+        );
+    }
+
+    public findLatestByHruidAndLanguage(hruid: string, language: Language) {
+        return this.findOne(
+            {
+                hruid: hruid,
+                language: language,
+            },
+            {
+                populate: ['keywords'],
+                orderBy: {
+                    version: 'DESC',
+                },
+            }
+        );
    }
-    // This repository is read-only for now since creating own learning object is an extension feature.
 }
--- a/backend/src/data/content/learning-path-repository.ts
+++ b/backend/src/data/content/learning-path-repository.ts
@ -4,7 +4,23 @@ import { Language } from '../../entities/content/language.js';

 export class LearningPathRepository extends DwengoEntityRepository<LearningPath> {
    public findByHruidAndLanguage(hruid: string, language: Language): Promise<LearningPath | null> {
-        return this.findOne({ hruid: hruid, language: language });
+        return this.findOne({ hruid: hruid, language: language }, { populate: ['nodes', 'nodes.transitions'] });
+    }
+
+    /**
+     * Returns all learning paths which have the given language and whose title OR description contains the
+     * query string.
+     *
+     * @param query The query string we want to seach for in the title or description.
+     * @param language The language of the learning paths we want to find.
+     */
+    public async findByQueryStringAndLanguage(query: string, language: Language): Promise<LearningPath[]> {
+        return this.findAll({
+            where: {
+                language: language,
+                $or: [{ title: { $like: `%${query}%` } }, { description: { $like: `%${query}%` } }],
+            },
+            populate: ['nodes', 'nodes.transitions'],
+        });
    }
-    // This repository is read-only for now since creating own learning object is an extension feature.
 }
--- a/backend/src/data/questions/answer-repository.ts
+++ b/backend/src/data/questions/answer-repository.ts
@ -5,10 +5,12 @@ import { Teacher } from '../../entities/users/teacher.entity.js';

 export class AnswerRepository extends DwengoEntityRepository<Answer> {
    public createAnswer(answer: { toQuestion: Question; author: Teacher; content: string }): Promise<Answer> {
-        const answerEntity = new Answer();
-        answerEntity.toQuestion = answer.toQuestion;
-        answerEntity.author = answer.author;
-        answerEntity.content = answer.content;
+        const answerEntity = this.create({
+            toQuestion: answer.toQuestion,
+            author: answer.author,
+            content: answer.content,
+            timestamp: new Date(),
+        });
        return this.insert(answerEntity);
    }
    public findAllAnswersToQuestion(question: Question): Promise<Answer[]> {
--- a/backend/src/data/questions/question-repository.ts
+++ b/backend/src/data/questions/question-repository.ts
@ -5,7 +5,14 @@ import { Student } from '../../entities/users/student.entity.js';

 export class QuestionRepository extends DwengoEntityRepository<Question> {
    public createQuestion(question: { loId: LearningObjectIdentifier; author: Student; content: string }): Promise<Question> {
-        const questionEntity = new Question();
+        const questionEntity = this.create({
+            learningObjectHruid: question.loId.hruid,
+            learningObjectLanguage: question.loId.language,
+            learningObjectVersion: question.loId.version,
+            author: question.author,
+            content: question.content,
+            timestamp: new Date(),
+        });
        questionEntity.learningObjectHruid = question.loId.hruid;
        questionEntity.learningObjectLanguage = question.loId.language;
        questionEntity.learningObjectVersion = question.loId.version;
--- a/backend/src/data/repositories.ts
+++ b/backend/src/data/repositories.ts
@ -28,6 +28,8 @@ import { LearningPath } from '../entities/content/learning-path.entity.js';
 import { LearningPathRepository } from './content/learning-path-repository.js';
 import { AttachmentRepository } from './content/attachment-repository.js';
 import { Attachment } from '../entities/content/attachment.entity.js';
+import { LearningPathNode } from '../entities/content/learning-path-node.entity.js';
+import { LearningPathTransition } from '../entities/content/learning-path-transition.entity.js';

 let entityManager: EntityManager | undefined;

@ -59,7 +61,7 @@ export const getTeacherRepository = repositoryGetter<Teacher, TeacherRepository>
 /* Classes */
 export const getClassRepository = repositoryGetter<Class, ClassRepository>(Class);
 export const getClassJoinRequestRepository = repositoryGetter<ClassJoinRequest, ClassJoinRequestRepository>(ClassJoinRequest);
-export const getTeacherInvitationRepository = repositoryGetter<TeacherInvitation, TeacherInvitationRepository>(TeacherInvitationRepository);
+export const getTeacherInvitationRepository = repositoryGetter<TeacherInvitation, TeacherInvitationRepository>(TeacherInvitation);

 /* Assignments */
 export const getAssignmentRepository = repositoryGetter<Assignment, AssignmentRepository>(Assignment);
@ -73,4 +75,6 @@ export const getAnswerRepository = repositoryGetter<Answer, AnswerRepository>(An
 /* Learning content */
 export const getLearningObjectRepository = repositoryGetter<LearningObject, LearningObjectRepository>(LearningObject);
 export const getLearningPathRepository = repositoryGetter<LearningPath, LearningPathRepository>(LearningPath);
-export const getAttachmentRepository = repositoryGetter<Attachment, AttachmentRepository>(Assignment);
+export const getLearningPathNodeRepository = repositoryGetter(LearningPathNode);
+export const getLearningPathTransitionRepository = repositoryGetter(LearningPathTransition);
+export const getAttachmentRepository = repositoryGetter<Attachment, AttachmentRepository>(Attachment);
--- a/backend/src/entities/assignments/assignment.entity.ts
+++ b/backend/src/entities/assignments/assignment.entity.ts
@ -2,8 +2,9 @@ import { Entity, Enum, ManyToOne, OneToMany, PrimaryKey, Property } from '@mikro
 import { Class } from '../classes/class.entity.js';
 import { Group } from './group.entity.js';
 import { Language } from '../content/language.js';
+import { AssignmentRepository } from '../../data/assignments/assignment-repository.js';

-@Entity()
+@Entity({ repository: () => AssignmentRepository })
 export class Assignment {
    @ManyToOne({ entity: () => Class, primary: true })
    within!: Class;
--- a/backend/src/entities/assignments/group.entity.ts
+++ b/backend/src/entities/assignments/group.entity.ts
@ -1,8 +1,9 @@
 import { Entity, ManyToMany, ManyToOne, PrimaryKey } from '@mikro-orm/core';
 import { Assignment } from './assignment.entity.js';
 import { Student } from '../users/student.entity.js';
+import { GroupRepository } from '../../data/assignments/group-repository.js';

-@Entity()
+@Entity({ repository: () => GroupRepository })
 export class Group {
    @ManyToOne({
        entity: () => Assignment,
--- a/backend/src/entities/assignments/submission.entity.ts
+++ b/backend/src/entities/assignments/submission.entity.ts
@ -2,8 +2,9 @@ import { Student } from '../users/student.entity.js';
 import { Group } from './group.entity.js';
 import { Entity, Enum, ManyToOne, PrimaryKey, Property } from '@mikro-orm/core';
 import { Language } from '../content/language.js';
+import { SubmissionRepository } from '../../data/assignments/submission-repository.js';

-@Entity()
+@Entity({ repository: () => SubmissionRepository })
 export class Submission {
    @PrimaryKey({ type: 'string' })
    learningObjectHruid!: string;
@ -14,8 +15,8 @@ export class Submission {
    })
    learningObjectLanguage!: Language;

-    @PrimaryKey({ type: 'string' })
-    learningObjectVersion: string = '1';
+    @PrimaryKey({ type: 'numeric' })
+    learningObjectVersion: number = 1;

    @PrimaryKey({ type: 'integer' })
    submissionNumber!: number;
--- a/backend/src/entities/classes/class-join-request.entity.ts
+++ b/backend/src/entities/classes/class-join-request.entity.ts
@ -1,8 +1,9 @@
 import { Entity, Enum, ManyToOne } from '@mikro-orm/core';
 import { Student } from '../users/student.entity.js';
 import { Class } from './class.entity.js';
+import { ClassJoinRequestRepository } from '../../data/classes/class-join-request-repository.js';

-@Entity()
+@Entity({ repository: () => ClassJoinRequestRepository })
 export class ClassJoinRequest {
    @ManyToOne({
        entity: () => Student,
--- a/backend/src/entities/classes/class.entity.ts
+++ b/backend/src/entities/classes/class.entity.ts
@ -2,8 +2,9 @@ import { Collection, Entity, ManyToMany, PrimaryKey, Property } from '@mikro-orm
 import { v4 } from 'uuid';
 import { Teacher } from '../users/teacher.entity.js';
 import { Student } from '../users/student.entity.js';
+import { ClassRepository } from '../../data/classes/class-repository.js';

-@Entity()
+@Entity({ repository: () => ClassRepository })
 export class Class {
    @PrimaryKey()
    classId = v4();
--- a/backend/src/entities/classes/teacher-invitation.entity.ts
+++ b/backend/src/entities/classes/teacher-invitation.entity.ts
@ -1,11 +1,15 @@
 import { Entity, ManyToOne } from '@mikro-orm/core';
 import { Teacher } from '../users/teacher.entity.js';
 import { Class } from './class.entity.js';
+import { TeacherInvitationRepository } from '../../data/classes/teacher-invitation-repository.js';

 /**
 * Invitation of a teacher into a class (in order to teach it).
 */
-@Entity()
+@Entity({ repository: () => TeacherInvitationRepository })
+@Entity({
+    repository: () => TeacherInvitationRepository,
+})
 export class TeacherInvitation {
    @ManyToOne({
        entity: () => Teacher,
--- a/backend/src/entities/content/attachment.entity.ts
+++ b/backend/src/entities/content/attachment.entity.ts
@ -1,7 +1,8 @@
 import { Entity, ManyToOne, PrimaryKey, Property } from '@mikro-orm/core';
 import { LearningObject } from './learning-object.entity.js';
+import { AttachmentRepository } from '../../data/content/attachment-repository.js';

-@Entity()
+@Entity({ repository: () => AttachmentRepository })
 export class Attachment {
    @ManyToOne({
        entity: () => LearningObject,
@ -9,8 +10,8 @@ export class Attachment {
    })
    learningObject!: LearningObject;

-    @PrimaryKey({ type: 'integer' })
-    sequenceNumber!: number;
+    @PrimaryKey({ type: 'string' })
+    name!: string;

    @Property({ type: 'string' })
    mimeType!: string;
--- a/backend/src/entities/content/language.ts
+++ b/backend/src/entities/content/language.ts
@ -1,6 +1,186 @@
 export enum Language {
+    Afar = 'aa',
+    Abkhazian = 'ab',
+    Afrikaans = 'af',
+    Akan = 'ak',
+    Albanian = 'sq',
+    Amharic = 'am',
+    Arabic = 'ar',
+    Aragonese = 'an',
+    Armenian = 'hy',
+    Assamese = 'as',
+    Avaric = 'av',
+    Avestan = 'ae',
+    Aymara = 'ay',
+    Azerbaijani = 'az',
+    Bashkir = 'ba',
+    Bambara = 'bm',
+    Basque = 'eu',
+    Belarusian = 'be',
+    Bengali = 'bn',
+    Bihari = 'bh',
+    Bislama = 'bi',
+    Bosnian = 'bs',
+    Breton = 'br',
+    Bulgarian = 'bg',
+    Burmese = 'my',
+    Catalan = 'ca',
+    Chamorro = 'ch',
+    Chechen = 'ce',
+    Chinese = 'zh',
+    ChurchSlavic = 'cu',
+    Chuvash = 'cv',
+    Cornish = 'kw',
+    Corsican = 'co',
+    Cree = 'cr',
+    Czech = 'cs',
+    Danish = 'da',
+    Divehi = 'dv',
    Dutch = 'nl',
-    French = 'fr',
+    Dzongkha = 'dz',
    English = 'en',
-    Germany = 'de',
+    Esperanto = 'eo',
+    Estonian = 'et',
+    Ewe = 'ee',
+    Faroese = 'fo',
+    Fijian = 'fj',
+    Finnish = 'fi',
+    French = 'fr',
+    Frisian = 'fy',
+    Fulah = 'ff',
+    Georgian = 'ka',
+    German = 'de',
+    Gaelic = 'gd',
+    Irish = 'ga',
+    Galician = 'gl',
+    Manx = 'gv',
+    Greek = 'el',
+    Guarani = 'gn',
+    Gujarati = 'gu',
+    Haitian = 'ht',
+    Hausa = 'ha',
+    Hebrew = 'he',
+    Herero = 'hz',
+    Hindi = 'hi',
+    HiriMotu = 'ho',
+    Croatian = 'hr',
+    Hungarian = 'hu',
+    Igbo = 'ig',
+    Icelandic = 'is',
+    Ido = 'io',
+    SichuanYi = 'ii',
+    Inuktitut = 'iu',
+    Interlingue = 'ie',
+    Interlingua = 'ia',
+    Indonesian = 'id',
+    Inupiaq = 'ik',
+    Italian = 'it',
+    Javanese = 'jv',
+    Japanese = 'ja',
+    Kalaallisut = 'kl',
+    Kannada = 'kn',
+    Kashmiri = 'ks',
+    Kanuri = 'kr',
+    Kazakh = 'kk',
+    Khmer = 'km',
+    Kikuyu = 'ki',
+    Kinyarwanda = 'rw',
+    Kirghiz = 'ky',
+    Komi = 'kv',
+    Kongo = 'kg',
+    Korean = 'ko',
+    Kuanyama = 'kj',
+    Kurdish = 'ku',
+    Lao = 'lo',
+    Latin = 'la',
+    Latvian = 'lv',
+    Limburgan = 'li',
+    Lingala = 'ln',
+    Lithuanian = 'lt',
+    Luxembourgish = 'lb',
+    LubaKatanga = 'lu',
+    Ganda = 'lg',
+    Macedonian = 'mk',
+    Marshallese = 'mh',
+    Malayalam = 'ml',
+    Maori = 'mi',
+    Marathi = 'mr',
+    Malay = 'ms',
+    Malagasy = 'mg',
+    Maltese = 'mt',
+    Mongolian = 'mn',
+    Nauru = 'na',
+    Navajo = 'nv',
+    SouthNdebele = 'nr',
+    NorthNdebele = 'nd',
+    Ndonga = 'ng',
+    Nepali = 'ne',
+    NorwegianNynorsk = 'nn',
+    NorwegianBokmal = 'nb',
+    Norwegian = 'no',
+    Chichewa = 'ny',
+    Occitan = 'oc',
+    Ojibwa = 'oj',
+    Oriya = 'or',
+    Oromo = 'om',
+    Ossetian = 'os',
+    Punjabi = 'pa',
+    Persian = 'fa',
+    Pali = 'pi',
+    Polish = 'pl',
+    Portuguese = 'pt',
+    Pashto = 'ps',
+    Quechua = 'qu',
+    Romansh = 'rm',
+    Romanian = 'ro',
+    Rundi = 'rn',
+    Russian = 'ru',
+    Sango = 'sg',
+    Sanskrit = 'sa',
+    Sinhala = 'si',
+    Slovak = 'sk',
+    Slovenian = 'sl',
+    NorthernSami = 'se',
+    Samoan = 'sm',
+    Shona = 'sn',
+    Sindhi = 'sd',
+    Somali = 'so',
+    Sotho = 'st',
+    Spanish = 'es',
+    Sardinian = 'sc',
+    Serbian = 'sr',
+    Swati = 'ss',
+    Sundanese = 'su',
+    Swahili = 'sw',
+    Swedish = 'sv',
+    Tahitian = 'ty',
+    Tamil = 'ta',
+    Tatar = 'tt',
+    Telugu = 'te',
+    Tajik = 'tg',
+    Tagalog = 'tl',
+    Thai = 'th',
+    Tibetan = 'bo',
+    Tigrinya = 'ti',
+    Tonga = 'to',
+    Tswana = 'tn',
+    Tsonga = 'ts',
+    Turkmen = 'tk',
+    Turkish = 'tr',
+    Twi = 'tw',
+    Uighur = 'ug',
+    Ukrainian = 'uk',
+    Urdu = 'ur',
+    Uzbek = 'uz',
+    Venda = 've',
+    Vietnamese = 'vi',
+    Volapuk = 'vo',
+    Welsh = 'cy',
+    Walloon = 'wa',
+    Wolof = 'wo',
+    Xhosa = 'xh',
+    Yiddish = 'yi',
+    Yoruba = 'yo',
+    Zhuang = 'za',
+    Zulu = 'zu',
 }
--- a/backend/src/entities/content/learning-object-identifier.ts
+++ b/backend/src/entities/content/learning-object-identifier.ts
@ -4,6 +4,6 @@ export class LearningObjectIdentifier {
    constructor(
        public hruid: string,
        public language: Language,
-        public version: string
+        public version: number
    ) {}
 }
--- a/backend/src/entities/content/learning-object.entity.ts
+++ b/backend/src/entities/content/learning-object.entity.ts
@ -2,6 +2,9 @@ import { Embeddable, Embedded, Entity, Enum, ManyToMany, OneToMany, PrimaryKey,
 import { Language } from './language.js';
 import { Attachment } from './attachment.entity.js';
 import { Teacher } from '../users/teacher.entity.js';
+import { DwengoContentType } from '../../services/learning-objects/processing/content-type.js';
+import { v4 } from 'uuid';
+import { LearningObjectRepository } from '../../data/content/learning-object-repository.js';

@Embeddable()
 export class EducationalGoal {
@ -21,7 +24,7 @@ export class ReturnValue {
    callbackSchema!: string;
 }

-@Entity()
+@Entity({ repository: () => LearningObjectRepository })
 export class LearningObject {
    @PrimaryKey({ type: 'string' })
    hruid!: string;
@ -32,8 +35,11 @@ export class LearningObject {
    })
    language!: Language;

-    @PrimaryKey({ type: 'string' })
-    version: string = '1';
+    @PrimaryKey({ type: 'number' })
+    version: number = 1;
+
+    @Property({ type: 'uuid', unique: true })
+    uuid = v4();

    @ManyToMany({
        entity: () => Teacher,
@ -47,19 +53,19 @@ export class LearningObject {
    description!: string;

    @Property({ type: 'string' })
-    contentType!: string;
+    contentType!: DwengoContentType;

    @Property({ type: 'array' })
    keywords: string[] = [];

    @Property({ type: 'array', nullable: true })
-    targetAges?: number[];
+    targetAges?: number[] = [];

    @Property({ type: 'bool' })
    teacherExclusive: boolean = false;

    @Property({ type: 'array' })
-    skosConcepts!: string[];
+    skosConcepts: string[] = [];

    @Embedded({
        entity: () => EducationalGoal,
@ -76,8 +82,8 @@ export class LearningObject {
    @Property({ type: 'smallint', nullable: true })
    difficulty?: number;

-    @Property({ type: 'integer' })
-    estimatedTime!: number;
+    @Property({ type: 'integer', nullable: true })
+    estimatedTime?: number;

    @Embedded({
        entity: () => ReturnValue,
@ -99,12 +105,3 @@ export class LearningObject {
    @Property({ type: 'blob' })
    content!: Buffer;
 }
-
-export enum ContentType {
-    Markdown = 'text/markdown',
-    Image = 'image/image',
-    Mpeg = 'audio/mpeg',
-    Pdf = 'application/pdf',
-    Extern = 'extern',
-    Blockly = 'Blockly',
-}
--- a/backend/src/entities/content/learning-path-node.entity.ts
+++ b/backend/src/entities/content/learning-path-node.entity.ts
@ -0,0 +1,37 @@
+import { Entity, Enum, ManyToOne, OneToMany, PrimaryKey, Property, Rel } from '@mikro-orm/core';
+import { Language } from './language.js';
+import { LearningPath } from './learning-path.entity.js';
+import { LearningPathTransition } from './learning-path-transition.entity.js';
+
+@Entity()
+export class LearningPathNode {
+    @ManyToOne({ entity: () => LearningPath, primary: true })
+    learningPath!: Rel<LearningPath>;
+
+    @PrimaryKey({ type: 'integer', autoincrement: true })
+    nodeNumber!: number;
+
+    @Property({ type: 'string' })
+    learningObjectHruid!: string;
+
+    @Enum({ items: () => Language })
+    language!: Language;
+
+    @Property({ type: 'number' })
+    version!: number;
+
+    @Property({ type: 'text', nullable: true })
+    instruction?: string;
+
+    @Property({ type: 'bool' })
+    startNode!: boolean;
+
+    @OneToMany({ entity: () => LearningPathTransition, mappedBy: 'node' })
+    transitions: LearningPathTransition[] = [];
+
+    @Property({ length: 3 })
+    createdAt: Date = new Date();
+
+    @Property({ length: 3, onUpdate: () => new Date() })
+    updatedAt: Date = new Date();
+}
--- a/backend/src/entities/content/learning-path-transition.entity.ts
+++ b/backend/src/entities/content/learning-path-transition.entity.ts
@ -0,0 +1,17 @@
+import { Entity, ManyToOne, PrimaryKey, Property, Rel } from '@mikro-orm/core';
+import { LearningPathNode } from './learning-path-node.entity.js';
+
+@Entity()
+export class LearningPathTransition {
+    @ManyToOne({ entity: () => LearningPathNode, primary: true })
+    node!: Rel<LearningPathNode>;
+
+    @PrimaryKey({ type: 'numeric' })
+    transitionNumber!: number;
+
+    @Property({ type: 'string' })
+    condition!: string;
+
+    @ManyToOne({ entity: () => LearningPathNode })
+    next!: Rel<LearningPathNode>;
+}
--- a/backend/src/entities/content/learning-path.entity.ts
+++ b/backend/src/entities/content/learning-path.entity.ts
@ -1,21 +1,18 @@
-import { Embeddable, Embedded, Entity, Enum, ManyToMany, OneToOne, PrimaryKey, Property } from '@mikro-orm/core';
+import { Entity, Enum, ManyToMany, OneToMany, PrimaryKey, Property } from '@mikro-orm/core';
 import { Language } from './language.js';
 import { Teacher } from '../users/teacher.entity.js';
+import { LearningPathRepository } from '../../data/content/learning-path-repository.js';
+import { LearningPathNode } from './learning-path-node.entity.js';

-@Entity()
+@Entity({ repository: () => LearningPathRepository })
 export class LearningPath {
    @PrimaryKey({ type: 'string' })
    hruid!: string;

-    @Enum({
-        items: () => Language,
-        primary: true,
-    })
+    @Enum({ items: () => Language, primary: true })
    language!: Language;

-    @ManyToMany({
-        entity: () => Teacher,
-    })
+    @ManyToMany({ entity: () => Teacher })
    admins!: Teacher[];

    @Property({ type: 'string' })
@ -24,49 +21,9 @@ export class LearningPath {
    @Property({ type: 'text' })
    description!: string;

-    @Property({ type: 'blob' })
-    image!: string;
+    @Property({ type: 'blob', nullable: true })
+    image: Buffer | null = null;

-    @Embedded({
-        entity: () => LearningPathNode,
-        array: true,
-    })
+    @OneToMany({ entity: () => LearningPathNode, mappedBy: 'learningPath' })
    nodes: LearningPathNode[] = [];
 }
-
-@Embeddable()
-export class LearningPathNode {
-    @Property({ type: 'string' })
-    learningObjectHruid!: string;
-
-    @Enum({
-        items: () => Language,
-    })
-    language!: Language;
-
-    @Property({ type: 'string' })
-    version!: string;
-
-    @Property({ type: 'longtext' })
-    instruction!: string;
-
-    @Property({ type: 'bool' })
-    startNode!: boolean;
-
-    @Embedded({
-        entity: () => LearningPathTransition,
-        array: true,
-    })
-    transitions!: LearningPathTransition[];
-}
-
-@Embeddable()
-export class LearningPathTransition {
-    @Property({ type: 'string' })
-    condition!: string;
-
-    @OneToOne({
-        entity: () => LearningPathNode,
-    })
-    next!: LearningPathNode;
-}
--- a/backend/src/entities/questions/answer.entity.ts
+++ b/backend/src/entities/questions/answer.entity.ts
@ -1,8 +1,9 @@
 import { Entity, ManyToOne, PrimaryKey, Property } from '@mikro-orm/core';
 import { Question } from './question.entity.js';
 import { Teacher } from '../users/teacher.entity.js';
+import { AnswerRepository } from '../../data/questions/answer-repository.js';

-@Entity()
+@Entity({ repository: () => AnswerRepository })
 export class Answer {
    @ManyToOne({
        entity: () => Teacher,
@ -16,8 +17,8 @@ export class Answer {
    })
    toQuestion!: Question;

-    @PrimaryKey({ type: 'integer' })
-    sequenceNumber!: number;
+    @PrimaryKey({ type: 'integer', autoincrement: true })
+    sequenceNumber?: number;

    @Property({ type: 'datetime' })
    timestamp: Date = new Date();
--- a/backend/src/entities/questions/question.entity.ts
+++ b/backend/src/entities/questions/question.entity.ts
@ -1,8 +1,9 @@
 import { Entity, Enum, ManyToOne, PrimaryKey, Property } from '@mikro-orm/core';
 import { Language } from '../content/language.js';
 import { Student } from '../users/student.entity.js';
+import { QuestionRepository } from '../../data/questions/question-repository.js';

-@Entity()
+@Entity({ repository: () => QuestionRepository })
 export class Question {
    @PrimaryKey({ type: 'string' })
    learningObjectHruid!: string;
@ -13,11 +14,11 @@ export class Question {
    })
    learningObjectLanguage!: Language;

-    @PrimaryKey({ type: 'string' })
-    learningObjectVersion: string = '1';
+    @PrimaryKey({ type: 'number' })
+    learningObjectVersion: number = 1;

-    @PrimaryKey({ type: 'integer' })
-    sequenceNumber!: number;
+    @PrimaryKey({ type: 'integer', autoincrement: true })
+    sequenceNumber?: number;

    @ManyToOne({
        entity: () => Student,
--- a/backend/src/entities/users/teacher.entity.ts
+++ b/backend/src/entities/users/teacher.entity.ts
@ -1,9 +1,18 @@
 import { Collection, Entity, ManyToMany } from '@mikro-orm/core';
 import { User } from './user.entity.js';
 import { Class } from '../classes/class.entity.js';
+import { TeacherRepository } from '../../data/users/teacher-repository.js';

-@Entity()
+@Entity({ repository: () => TeacherRepository })
 export class Teacher extends User {
    @ManyToMany(() => Class)
    classes!: Collection<Class>;
+
+    constructor(
+        public username: string,
+        public firstName: string,
+        public lastName: string
+    ) {
+        super();
+    }
 }
--- a/backend/src/exceptions.ts
+++ b/backend/src/exceptions.ts
@ -1,3 +1,17 @@
+/**
+ * Exception for HTTP 400 Bad Request
+ */
+export class BadRequestException extends Error {
+    public status = 400;
+
+    constructor(error: string) {
+        super(error);
+    }
+}
+
+/**
+ * Exception for HTTP 401 Unauthorized
+ */
 export class UnauthorizedException extends Error {
    status = 401;
    constructor(message: string = 'Unauthorized') {
@ -5,9 +19,24 @@ export class UnauthorizedException extends Error {
    }
 }

+/**
+ * Exception for HTTP 403 Forbidden
+ */
 export class ForbiddenException extends Error {
    status = 403;
+
    constructor(message: string = 'Forbidden') {
        super(message);
    }
 }
+
+/**
+ * Exception for HTTP 404 Not Found
+ */
+export class NotFoundException extends Error {
+    public status = 404;
+
+    constructor(error: string) {
+        super(error);
+    }
+}
--- a/backend/src/interfaces/learning-content.ts
+++ b/backend/src/interfaces/learning-content.ts
@ -1,3 +1,5 @@
+import { Language } from '../entities/content/language';
+
 export interface Transition {
    default: boolean;
    _id: string;
@ -9,15 +11,22 @@ export interface Transition {
    };
 }

+export interface LearningObjectIdentifier {
+    hruid: string;
+    language: Language;
+    version?: number;
+}
+
 export interface LearningObjectNode {
    _id: string;
    learningobject_hruid: string;
    version: number;
-    language: string;
+    language: Language;
    start_node?: boolean;
    transitions: Transition[];
    created_at: string;
    updatedAt: string;
+    done?: boolean; // True if a submission exists for this node by the user for whom the learning path is customized.
 }

 export interface LearningPath {
@ -37,6 +46,11 @@ export interface LearningPath {
    __order: number;
 }

+export interface LearningPathIdentifier {
+    hruid: string;
+    language: Language;
+}
+
 export interface EducationalGoal {
    source: string;
    id: string;
@ -52,7 +66,7 @@ export interface LearningObjectMetadata {
    uuid: string;
    hruid: string;
    version: number;
-    language: string;
+    language: Language;
    title: string;
    description: string;
    difficulty: number;
@ -75,9 +89,9 @@ export interface FilteredLearningObject {
    version: number;
    title: string;
    htmlUrl: string;
-    language: string;
+    language: Language;
    difficulty: number;
-    estimatedTime: number;
+    estimatedTime?: number;
    available: boolean;
    teacherExclusive: boolean;
    educationalGoals: EducationalGoal[];
--- a/backend/src/mikro-orm.config.ts
+++ b/backend/src/mikro-orm.config.ts
@ -24,6 +24,7 @@ import { LearningPath } from './entities/content/learning-path.entity.js';

 import { Answer } from './entities/questions/answer.entity.js';
 import { Question } from './entities/questions/question.entity.js';
+import { SqliteAutoincrementSubscriber } from './sqlite-autoincrement-workaround.js';

 const entities = [
    User,
@ -47,6 +48,7 @@ function config(testingMode: boolean = false): Options {
        return {
            driver: SqliteDriver,
            dbName: getEnvVar(EnvVars.DbName),
+            subscribers: [new SqliteAutoincrementSubscriber()],
            entities: entities,
            // EntitiesTs: entitiesTs,

--- a/backend/src/routes/learning-objects.ts
+++ b/backend/src/routes/learning-objects.ts
@ -1,5 +1,5 @@
 import express from 'express';
-import { getAllLearningObjects, getLearningObject } from '../controllers/learningObjects.js';
+import { getAllLearningObjects, getAttachment, getLearningObject, getLearningObjectHTML } from '../controllers/learning-objects.js';

 const router = express.Router();

@ -21,4 +21,16 @@ router.get('/', getAllLearningObjects);
 // Example: http://localhost:3000/learningObject/un_ai7
 router.get('/:hruid', getLearningObject);

+// Parameter: hruid of learning object
+// Query: language, version (optional)
+// Route to fetch the HTML rendering of one learning object based on its hruid.
+// Example: http://localhost:3000/learningObject/un_ai7/html
+router.get('/:hruid/html', getLearningObjectHTML);
+
+// Parameter: hruid of learning object, name of attachment.
+// Query: language, version (optional).
+// Route to get the raw data of the attachment for one learning object based on its hruid.
+// Example: http://localhost:3000/learningObject/u_test/attachment/testimage.png
+router.get('/:hruid/html/:attachmentName', getAttachment);
+
 export default router;
--- a/backend/src/routes/learning-paths.ts
+++ b/backend/src/routes/learning-paths.ts
@ -1,5 +1,5 @@
 import express from 'express';
-import { getLearningPaths } from '../controllers/learningPaths.js';
+import { getLearningPaths } from '../controllers/learning-paths.js';

 const router = express.Router();

--- a/backend/src/services/learning-objects/attachment-service.ts
+++ b/backend/src/services/learning-objects/attachment-service.ts
@ -0,0 +1,23 @@
+import { getAttachmentRepository } from '../../data/repositories.js';
+import { Attachment } from '../../entities/content/attachment.entity.js';
+import { LearningObjectIdentifier } from '../../interfaces/learning-content.js';
+
+const attachmentService = {
+    getAttachment(learningObjectId: LearningObjectIdentifier, attachmentName: string): Promise<Attachment | null> {
+        const attachmentRepo = getAttachmentRepository();
+
+        if (learningObjectId.version) {
+            return attachmentRepo.findByLearningObjectIdAndName(
+                {
+                    hruid: learningObjectId.hruid,
+                    language: learningObjectId.language,
+                    version: learningObjectId.version,
+                },
+                attachmentName
+            );
+        }
+        return attachmentRepo.findByMostRecentVersionOfLearningObjectAndName(learningObjectId.hruid, learningObjectId.language, attachmentName);
+    },
+};
+
+export default attachmentService;
--- a/backend/src/services/learning-objects/database-learning-object-provider.ts
+++ b/backend/src/services/learning-objects/database-learning-object-provider.ts
@ -0,0 +1,115 @@
+import { LearningObjectProvider } from './learning-object-provider.js';
+import { FilteredLearningObject, LearningObjectIdentifier, LearningPathIdentifier } from '../../interfaces/learning-content.js';
+import { getLearningObjectRepository, getLearningPathRepository } from '../../data/repositories.js';
+import { Language } from '../../entities/content/language.js';
+import { LearningObject } from '../../entities/content/learning-object.entity.js';
+import { getUrlStringForLearningObject } from '../../util/links.js';
+import processingService from './processing/processing-service.js';
+import { NotFoundError } from '@mikro-orm/core';
+import learningObjectService from './learning-object-service.js';
+import { getLogger, Logger } from '../../logging/initalize.js';
+
+const logger: Logger = getLogger();
+
+function convertLearningObject(learningObject: LearningObject | null): FilteredLearningObject | null {
+    if (!learningObject) {
+        return null;
+    }
+    return {
+        key: learningObject.hruid,
+        _id: learningObject.uuid, // For backwards compatibility with the original Dwengo API, we also populate the _id field.
+        uuid: learningObject.uuid,
+        language: learningObject.language,
+        version: learningObject.version,
+        title: learningObject.title,
+        description: learningObject.description,
+        htmlUrl: getUrlStringForLearningObject(learningObject),
+        available: learningObject.available,
+        contentType: learningObject.contentType,
+        contentLocation: learningObject.contentLocation,
+        difficulty: learningObject.difficulty || 1,
+        estimatedTime: learningObject.estimatedTime,
+        keywords: learningObject.keywords,
+        educationalGoals: learningObject.educationalGoals,
+        returnValue: {
+            callback_url: learningObject.returnValue.callbackUrl,
+            callback_schema: JSON.parse(learningObject.returnValue.callbackSchema),
+        },
+        skosConcepts: learningObject.skosConcepts,
+        targetAges: learningObject.targetAges || [],
+        teacherExclusive: learningObject.teacherExclusive,
+    };
+}
+
+function findLearningObjectEntityById(id: LearningObjectIdentifier): Promise<LearningObject | null> {
+    const learningObjectRepo = getLearningObjectRepository();
+
+    return learningObjectRepo.findLatestByHruidAndLanguage(id.hruid, id.language as Language);
+}
+
+/**
+ * Service providing access to data about learning objects from the database
+ */
+const databaseLearningObjectProvider: LearningObjectProvider = {
+    /**
+     * Fetches a single learning object by its HRUID
+     */
+    async getLearningObjectById(id: LearningObjectIdentifier): Promise<FilteredLearningObject | null> {
+        const learningObject = await findLearningObjectEntityById(id);
+        return convertLearningObject(learningObject);
+    },
+
+    /**
+     * Obtain a HTML-rendering of the learning object with the given identifier (as a string).
+     */
+    async getLearningObjectHTML(id: LearningObjectIdentifier): Promise<string | null> {
+        const learningObjectRepo = getLearningObjectRepository();
+
+        const learningObject = await learningObjectRepo.findLatestByHruidAndLanguage(id.hruid, id.language as Language);
+        if (!learningObject) {
+            return null;
+        }
+        return await processingService.render(learningObject, (id) => findLearningObjectEntityById(id));
+    },
+
+    /**
+     * Fetch the HRUIDs of all learning objects on this path.
+     */
+    async getLearningObjectIdsFromPath(id: LearningPathIdentifier): Promise<string[]> {
+        const learningPathRepo = getLearningPathRepository();
+
+        const learningPath = await learningPathRepo.findByHruidAndLanguage(id.hruid, id.language);
+        if (!learningPath) {
+            throw new NotFoundError('The learning path with the given ID could not be found.');
+        }
+        return learningPath.nodes.map((it) => it.learningObjectHruid); // TODO: Determine this based on the submissions of the user.
+    },
+
+    /**
+     * Fetch the full metadata of all learning objects on this path.
+     */
+    async getLearningObjectsFromPath(id: LearningPathIdentifier): Promise<FilteredLearningObject[]> {
+        const learningPathRepo = getLearningPathRepository();
+
+        const learningPath = await learningPathRepo.findByHruidAndLanguage(id.hruid, id.language);
+        if (!learningPath) {
+            throw new NotFoundError('The learning path with the given ID could not be found.');
+        }
+        const learningObjects = await Promise.all(
+            learningPath.nodes.map((it) => {
+                const learningObject = learningObjectService.getLearningObjectById({
+                    hruid: it.learningObjectHruid,
+                    language: it.language,
+                    version: it.version,
+                });
+                if (learningObject === null) {
+                    logger.warn(`WARN: Learning object corresponding with node ${it} not found!`);
+                }
+                return learningObject;
+            })
+        );
+        return learningObjects.filter((it) => it !== null);
+    },
+};
+
+export default databaseLearningObjectProvider;
--- a/backend/src/services/learning-objects/dwengo-api-learning-object-provider.ts
+++ b/backend/src/services/learning-objects/dwengo-api-learning-object-provider.ts
@ -0,0 +1,138 @@
+import { DWENGO_API_BASE } from '../../config.js';
+import { fetchWithLogging } from '../../util/apiHelper.js';
+import {
+    FilteredLearningObject,
+    LearningObjectIdentifier,
+    LearningObjectMetadata,
+    LearningObjectNode,
+    LearningPathIdentifier,
+    LearningPathResponse,
+} from '../../interfaces/learning-content.js';
+import dwengoApiLearningPathProvider from '../learning-paths/dwengo-api-learning-path-provider.js';
+import { LearningObjectProvider } from './learning-object-provider.js';
+import { getLogger, Logger } from '../../logging/initalize.js';
+
+const logger: Logger = getLogger();
+
+/**
+ * Helper function to convert the learning object metadata retrieved from the API to a FilteredLearningObject which
+ * our API should return.
+ * @param data
+ */
+function filterData(data: LearningObjectMetadata): FilteredLearningObject {
+    return {
+        key: data.hruid, // Hruid learningObject (not path)
+        _id: data._id,
+        uuid: data.uuid,
+        version: data.version,
+        title: data.title,
+        htmlUrl: `/learningObject/${data.hruid}/html?language=${data.language}&version=${data.version}`, // Url to fetch html content
+        language: data.language,
+        difficulty: data.difficulty,
+        estimatedTime: data.estimated_time,
+        available: data.available,
+        teacherExclusive: data.teacher_exclusive,
+        educationalGoals: data.educational_goals, // List with learningObjects
+        keywords: data.keywords, // For search
+        description: data.description, // For search (not an actual description)
+        targetAges: data.target_ages,
+        contentType: data.content_type, // Markdown, image, audio, etc.
+        contentLocation: data.content_location, // If content type extern
+        skosConcepts: data.skos_concepts,
+        returnValue: data.return_value, // Callback response information
+    };
+}
+
+/**
+ * Generic helper function to fetch all learning objects from a given path (full data or just HRUIDs)
+ */
+async function fetchLearningObjects(learningPathId: LearningPathIdentifier, full: boolean): Promise<FilteredLearningObject[] | string[]> {
+    try {
+        const learningPathResponse: LearningPathResponse = await dwengoApiLearningPathProvider.fetchLearningPaths(
+            [learningPathId.hruid],
+            learningPathId.language,
+            `Learning path for HRUID "${learningPathId.hruid}"`
+        );
+
+        if (!learningPathResponse.success || !learningPathResponse.data?.length) {
+            logger.warn(`⚠️ WARNING: Learning path "${learningPathId.hruid}" exists but contains no learning objects.`);
+            return [];
+        }
+
+        const nodes: LearningObjectNode[] = learningPathResponse.data[0].nodes;
+
+        if (!full) {
+            return nodes.map((node) => node.learningobject_hruid);
+        }
+
+        const objects = await Promise.all(
+            nodes.map(async (node) =>
+                dwengoApiLearningObjectProvider.getLearningObjectById({
+                    hruid: node.learningobject_hruid,
+                    language: learningPathId.language,
+                })
+            )
+        );
+        return objects.filter((obj): obj is FilteredLearningObject => obj !== null);
+    } catch (error) {
+        logger.error('❌ Error fetching learning objects:', error);
+        return [];
+    }
+}
+
+const dwengoApiLearningObjectProvider: LearningObjectProvider = {
+    /**
+     * Fetches a single learning object by its HRUID
+     */
+    async getLearningObjectById(id: LearningObjectIdentifier): Promise<FilteredLearningObject | null> {
+        const metadataUrl = `${DWENGO_API_BASE}/learningObject/getMetadata`;
+        const metadata = await fetchWithLogging<LearningObjectMetadata>(
+            metadataUrl,
+            `Metadata for Learning Object HRUID "${id.hruid}" (language ${id.language})`,
+            {
+                params: id,
+            }
+        );
+
+        if (!metadata || typeof metadata !== 'object') {
+            logger.warn(`⚠️ WARNING: Learning object "${id.hruid}" not found.`);
+            return null;
+        }
+
+        return filterData(metadata);
+    },
+
+    /**
+     * Fetch full learning object data (metadata)
+     */
+    async getLearningObjectsFromPath(id: LearningPathIdentifier): Promise<FilteredLearningObject[]> {
+        return (await fetchLearningObjects(id, true)) as FilteredLearningObject[];
+    },
+
+    /**
+     * Fetch only learning object HRUIDs
+     */
+    async getLearningObjectIdsFromPath(id: LearningPathIdentifier): Promise<string[]> {
+        return (await fetchLearningObjects(id, false)) as string[];
+    },
+
+    /**
+     * Obtain a HTML-rendering of the learning object with the given identifier (as a string). For learning objects
+     * from the Dwengo API, this means passing through the HTML rendering from there.
+     */
+    async getLearningObjectHTML(id: LearningObjectIdentifier): Promise<string | null> {
+        const htmlUrl = `${DWENGO_API_BASE}/learningObject/getRaw`;
+        const html = await fetchWithLogging<string>(htmlUrl, `Metadata for Learning Object HRUID "${id.hruid}" (language ${id.language})`, {
+            params: id,
+        });
+
+        if (!html) {
+            logger.warn(`⚠️ WARNING: Learning object "${id.hruid}" not found.`);
+            return null;
+        }
+
+        return html;
+    },
+};
+
+export default dwengoApiLearningObjectProvider;
--- a/backend/src/services/learning-objects/learning-object-provider.ts
+++ b/backend/src/services/learning-objects/learning-object-provider.ts
@ -0,0 +1,23 @@
+import { FilteredLearningObject, LearningObjectIdentifier, LearningPathIdentifier } from '../../interfaces/learning-content.js';
+
+export interface LearningObjectProvider {
+    /**
+     * Fetches a single learning object by its HRUID
+     */
+    getLearningObjectById(id: LearningObjectIdentifier): Promise<FilteredLearningObject | null>;
+
+    /**
+     * Fetch full learning object data (metadata)
+     */
+    getLearningObjectsFromPath(id: LearningPathIdentifier): Promise<FilteredLearningObject[]>;
+
+    /**
+     * Fetch only learning object HRUIDs
+     */
+    getLearningObjectIdsFromPath(id: LearningPathIdentifier): Promise<string[]>;
+
+    /**
+     * Obtain a HTML-rendering of the learning object with the given identifier (as a string).
+     */
+    getLearningObjectHTML(id: LearningObjectIdentifier): Promise<string | null>;
+}
--- a/backend/src/services/learning-objects/learning-object-service.ts
+++ b/backend/src/services/learning-objects/learning-object-service.ts
@ -0,0 +1,47 @@
+import { FilteredLearningObject, LearningObjectIdentifier, LearningPathIdentifier } from '../../interfaces/learning-content.js';
+import dwengoApiLearningObjectProvider from './dwengo-api-learning-object-provider.js';
+import { LearningObjectProvider } from './learning-object-provider.js';
+import { EnvVars, getEnvVar } from '../../util/envvars.js';
+import databaseLearningObjectProvider from './database-learning-object-provider.js';
+
+function getProvider(id: LearningObjectIdentifier): LearningObjectProvider {
+    if (id.hruid.startsWith(getEnvVar(EnvVars.UserContentPrefix))) {
+        return databaseLearningObjectProvider;
+    }
+    return dwengoApiLearningObjectProvider;
+}
+
+/**
+ * Service providing access to data about learning objects from the appropriate data source (database or Dwengo-api)
+ */
+const learningObjectService = {
+    /**
+     * Fetches a single learning object by its HRUID
+     */
+    getLearningObjectById(id: LearningObjectIdentifier): Promise<FilteredLearningObject | null> {
+        return getProvider(id).getLearningObjectById(id);
+    },
+
+    /**
+     * Fetch full learning object data (metadata)
+     */
+    getLearningObjectsFromPath(id: LearningPathIdentifier): Promise<FilteredLearningObject[]> {
+        return getProvider(id).getLearningObjectsFromPath(id);
+    },
+
+    /**
+     * Fetch only learning object HRUIDs
+     */
+    getLearningObjectIdsFromPath(id: LearningPathIdentifier): Promise<string[]> {
+        return getProvider(id).getLearningObjectIdsFromPath(id);
+    },
+
+    /**
+     * Obtain a HTML-rendering of the learning object with the given identifier (as a string).
+     */
+    getLearningObjectHTML(id: LearningObjectIdentifier): Promise<string | null> {
+        return getProvider(id).getLearningObjectHTML(id);
+    },
+};
+
+export default learningObjectService;
--- a/backend/src/services/learning-objects/processing/audio/audio-processor.ts
+++ b/backend/src/services/learning-objects/processing/audio/audio-processor.ts
@ -0,0 +1,25 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/audio/audio_processor.js
+ *
+ * WARNING: The support for audio learning objects is currently still experimental.
+ */
+
+import DOMPurify from 'isomorphic-dompurify';
+import { type } from 'node:os';
+import { DwengoContentType } from '../content-type.js';
+import { StringProcessor } from '../string-processor.js';
+
+class AudioProcessor extends StringProcessor {
+    constructor() {
+        super(DwengoContentType.AUDIO_MPEG);
+    }
+
+    protected renderFn(audioUrl: string): string {
+        return DOMPurify.sanitize(`<audio controls>
+            <source src="${audioUrl}" type=${type}>
+            Your browser does not support the audio element.
+            </audio>`);
+    }
+}
+
+export default AudioProcessor;
--- a/backend/src/services/learning-objects/processing/content-type.ts
+++ b/backend/src/services/learning-objects/processing/content-type.ts
@ -0,0 +1,18 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/content_type.js
+ */
+
+enum DwengoContentType {
+    TEXT_PLAIN = 'text/plain',
+    TEXT_MARKDOWN = 'text/markdown',
+    IMAGE_BLOCK = 'image/image-block',
+    IMAGE_INLINE = 'image/image',
+    AUDIO_MPEG = 'audio/mpeg',
+    APPLICATION_PDF = 'application/pdf',
+    EXTERN = 'extern',
+    BLOCKLY = 'blockly',
+    GIFT = 'text/gift',
+    CT_SCHEMA = 'text/ct-schema',
+}
+
+export { DwengoContentType };
--- a/backend/src/services/learning-objects/processing/extern/extern-processor.ts
+++ b/backend/src/services/learning-objects/processing/extern/extern-processor.ts
@ -0,0 +1,40 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/extern/extern_processor.js
+ *
+ * WARNING: The support for external content is currently still experimental.
+ */
+
+import DOMPurify from 'isomorphic-dompurify';
+import { ProcessingError } from '../processing-error.js';
+import { isValidHttpUrl } from '../../../../util/links.js';
+import { DwengoContentType } from '../content-type.js';
+import { StringProcessor } from '../string-processor.js';
+
+class ExternProcessor extends StringProcessor {
+    constructor() {
+        super(DwengoContentType.EXTERN);
+    }
+
+    override renderFn(externURL: string) {
+        if (!isValidHttpUrl(externURL)) {
+            throw new ProcessingError('The url is not valid: ' + externURL);
+        }
+
+        // If a seperate youtube-processor would be added, this code would need to move to that processor
+        // Converts youtube urls to youtube-embed urls
+        const match = /(.*youtube.com\/)watch\?v=(.*)/.exec(externURL);
+        if (match) {
+            externURL = match[1] + 'embed/' + match[2];
+        }
+
+        return DOMPurify.sanitize(
+            `
+            <div class="iframe-container">
+                <iframe src="${externURL}" allowfullscreen></iframe>
+            </div>`,
+            { ADD_TAGS: ['iframe'], ADD_ATTR: ['allow', 'allowfullscreen', 'frameborder', 'scrolling'] }
+        );
+    }
+}
+
+export default ExternProcessor;
--- a/backend/src/services/learning-objects/processing/gift/gift-processor.ts
+++ b/backend/src/services/learning-objects/processing/gift/gift-processor.ts
@ -0,0 +1,61 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/gift/gift_processor.js
+ */
+
+import DOMPurify from 'isomorphic-dompurify';
+import { GIFTQuestion, parse } from 'gift-pegjs';
+import { DwengoContentType } from '../content-type.js';
+import { GIFTQuestionRenderer } from './question-renderers/gift-question-renderer.js';
+import { MultipleChoiceQuestionRenderer } from './question-renderers/multiple-choice-question-renderer.js';
+import { CategoryQuestionRenderer } from './question-renderers/category-question-renderer.js';
+import { DescriptionQuestionRenderer } from './question-renderers/description-question-renderer.js';
+import { EssayQuestionRenderer } from './question-renderers/essay-question-renderer.js';
+import { MatchingQuestionRenderer } from './question-renderers/matching-question-renderer.js';
+import { NumericalQuestionRenderer } from './question-renderers/numerical-question-renderer.js';
+import { ShortQuestionRenderer } from './question-renderers/short-question-renderer.js';
+import { TrueFalseQuestionRenderer } from './question-renderers/true-false-question-renderer.js';
+import { StringProcessor } from '../string-processor.js';
+
+class GiftProcessor extends StringProcessor {
+    private renderers: RendererMap = {
+        Category: new CategoryQuestionRenderer(),
+        Description: new DescriptionQuestionRenderer(),
+        Essay: new EssayQuestionRenderer(),
+        Matching: new MatchingQuestionRenderer(),
+        Numerical: new NumericalQuestionRenderer(),
+        Short: new ShortQuestionRenderer(),
+        TF: new TrueFalseQuestionRenderer(),
+        MC: new MultipleChoiceQuestionRenderer(),
+    };
+
+    constructor() {
+        super(DwengoContentType.GIFT);
+    }
+
+    override renderFn(giftString: string) {
+        const quizQuestions: GIFTQuestion[] = parse(giftString);
+
+        let html = "<div class='learning-object-gift'>\n";
+        let i = 1;
+        for (const question of quizQuestions) {
+            html += `    <div class='gift-question' id='gift-q${i}'>\n`;
+            html += '        ' + this.renderQuestion(question, i).replaceAll(/\n(.+)/g, '\n        $1'); // Replace for indentation.
+            html += `    </div>\n`;
+            i++;
+        }
+        html += '</div>\n';
+
+        return DOMPurify.sanitize(html);
+    }
+
+    private renderQuestion<T extends GIFTQuestion>(question: T, questionNumber: number): string {
+        const renderer = this.renderers[question.type] as GIFTQuestionRenderer<T>;
+        return renderer.render(question, questionNumber);
+    }
+}
+
+type RendererMap = {
+    [K in GIFTQuestion['type']]: GIFTQuestionRenderer<Extract<GIFTQuestion, { type: K }>>;
+};
+
+export default GiftProcessor;
--- a/backend/src/services/learning-objects/processing/gift/question-renderers/category-question-renderer.ts
+++ b/backend/src/services/learning-objects/processing/gift/question-renderers/category-question-renderer.ts
@ -0,0 +1,9 @@
+import { GIFTQuestionRenderer } from './gift-question-renderer.js';
+import { Category } from 'gift-pegjs';
+import { ProcessingError } from '../../processing-error.js';
+
+export class CategoryQuestionRenderer extends GIFTQuestionRenderer<Category> {
+    render(question: Category, questionNumber: number): string {
+        throw new ProcessingError("The question type 'Category' is not supported yet!");
+    }
+}
--- a/backend/src/services/learning-objects/processing/gift/question-renderers/description-question-renderer.ts
+++ b/backend/src/services/learning-objects/processing/gift/question-renderers/description-question-renderer.ts
@ -0,0 +1,9 @@
+import { GIFTQuestionRenderer } from './gift-question-renderer.js';
+import { Description } from 'gift-pegjs';
+import { ProcessingError } from '../../processing-error.js';
+
+export class DescriptionQuestionRenderer extends GIFTQuestionRenderer<Description> {
+    render(question: Description, questionNumber: number): string {
+        throw new ProcessingError("The question type 'Description' is not supported yet!");
+    }
+}
--- a/backend/src/services/learning-objects/processing/gift/question-renderers/essay-question-renderer.ts
+++ b/backend/src/services/learning-objects/processing/gift/question-renderers/essay-question-renderer.ts
@ -0,0 +1,16 @@
+import { GIFTQuestionRenderer } from './gift-question-renderer.js';
+import { Essay } from 'gift-pegjs';
+
+export class EssayQuestionRenderer extends GIFTQuestionRenderer<Essay> {
+    render(question: Essay, questionNumber: number): string {
+        let renderedHtml = '';
+        if (question.title) {
+            renderedHtml += `<h2 class='gift-title' id='gift-q${questionNumber}-title'>${question.title}</h2>\n`;
+        }
+        if (question.stem) {
+            renderedHtml += `<p class='gift-stem' id='gift-q${questionNumber}-stem'>${question.stem.text}</p>\n`;
+        }
+        renderedHtml += `<textarea class='gift-essay-answer' id='gift-q${questionNumber}-answer'></textarea>\n`;
+        return renderedHtml;
+    }
+}
--- a/backend/src/services/learning-objects/processing/gift/question-renderers/gift-question-renderer.ts
+++ b/backend/src/services/learning-objects/processing/gift/question-renderers/gift-question-renderer.ts
@ -0,0 +1,14 @@
+import { GIFTQuestion } from 'gift-pegjs';
+
+/**
+ * Subclasses of this class are renderers which can render a specific type of GIFT questions to HTML.
+ */
+export abstract class GIFTQuestionRenderer<T extends GIFTQuestion> {
+    /**
+     * Render the given question to HTML.
+     * @param question The question.
+     * @param questionNumber The index number of the question.
+     * @returns The question rendered as HTML.
+     */
+    abstract render(question: T, questionNumber: number): string;
+}
--- a/backend/src/services/learning-objects/processing/gift/question-renderers/matching-question-renderer.ts
+++ b/backend/src/services/learning-objects/processing/gift/question-renderers/matching-question-renderer.ts
@ -0,0 +1,9 @@
+import { GIFTQuestionRenderer } from './gift-question-renderer.js';
+import { Matching } from 'gift-pegjs';
+import { ProcessingError } from '../../processing-error.js';
+
+export class MatchingQuestionRenderer extends GIFTQuestionRenderer<Matching> {
+    render(question: Matching, questionNumber: number): string {
+        throw new ProcessingError("The question type 'Matching' is not supported yet!");
+    }
+}
--- a/backend/src/services/learning-objects/processing/gift/question-renderers/multiple-choice-question-renderer.ts
+++ b/backend/src/services/learning-objects/processing/gift/question-renderers/multiple-choice-question-renderer.ts
@ -0,0 +1,23 @@
+import { GIFTQuestionRenderer } from './gift-question-renderer.js';
+import { MultipleChoice } from 'gift-pegjs';
+
+export class MultipleChoiceQuestionRenderer extends GIFTQuestionRenderer<MultipleChoice> {
+    render(question: MultipleChoice, questionNumber: number): string {
+        let renderedHtml = '';
+        if (question.title) {
+            renderedHtml += `<h2 class='gift-title' id='gift-q${questionNumber}-title'>${question.title}</h2>\n`;
+        }
+        if (question.stem) {
+            renderedHtml += `<p class='gift-stem' id='gift-q${questionNumber}-stem'>${question.stem.text}</p>\n`;
+        }
+        let i = 0;
+        for (const choice of question.choices) {
+            renderedHtml += `<div class="gift-choice-div">\n`;
+            renderedHtml += `    <input type='radio' id='gift-q${questionNumber}-choice-${i}' name='gift-q${questionNumber}-choices' value="${i}"/>\n`;
+            renderedHtml += `    <label for='gift-q${questionNumber}-choice-${i}'>${choice.text}</label>\n`;
+            renderedHtml += `</div>\n`;
+            i++;
+        }
+        return renderedHtml;
+    }
+}
--- a/backend/src/services/learning-objects/processing/gift/question-renderers/numerical-question-renderer.ts
+++ b/backend/src/services/learning-objects/processing/gift/question-renderers/numerical-question-renderer.ts
@ -0,0 +1,9 @@
+import { GIFTQuestionRenderer } from './gift-question-renderer.js';
+import { Numerical } from 'gift-pegjs';
+import { ProcessingError } from '../../processing-error.js';
+
+export class NumericalQuestionRenderer extends GIFTQuestionRenderer<Numerical> {
+    render(question: Numerical, questionNumber: number): string {
+        throw new ProcessingError("The question type 'Numerical' is not supported yet!");
+    }
+}
--- a/backend/src/services/learning-objects/processing/gift/question-renderers/short-question-renderer.ts
+++ b/backend/src/services/learning-objects/processing/gift/question-renderers/short-question-renderer.ts
@ -0,0 +1,9 @@
+import { GIFTQuestionRenderer } from './gift-question-renderer.js';
+import { ShortAnswer } from 'gift-pegjs';
+import { ProcessingError } from '../../processing-error.js';
+
+export class ShortQuestionRenderer extends GIFTQuestionRenderer<ShortAnswer> {
+    render(question: ShortAnswer, questionNumber: number): string {
+        throw new ProcessingError("The question type 'ShortAnswer' is not supported yet!");
+    }
+}
--- a/backend/src/services/learning-objects/processing/gift/question-renderers/true-false-question-renderer.ts
+++ b/backend/src/services/learning-objects/processing/gift/question-renderers/true-false-question-renderer.ts
@ -0,0 +1,9 @@
+import { GIFTQuestionRenderer } from './gift-question-renderer.js';
+import { TrueFalse } from 'gift-pegjs';
+import { ProcessingError } from '../../processing-error.js';
+
+export class TrueFalseQuestionRenderer extends GIFTQuestionRenderer<TrueFalse> {
+    render(question: TrueFalse, questionNumber: number): string {
+        throw new ProcessingError("The question type 'TrueFalse' is not supported yet!");
+    }
+}
--- a/backend/src/services/learning-objects/processing/image/block-image-processor.ts
+++ b/backend/src/services/learning-objects/processing/image/block-image-processor.ts
@ -0,0 +1,19 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/image/block_image_processor.js
+ */
+
+import InlineImageProcessor from './inline-image-processor.js';
+import DOMPurify from 'isomorphic-dompurify';
+
+class BlockImageProcessor extends InlineImageProcessor {
+    constructor() {
+        super();
+    }
+
+    override renderFn(imageUrl: string) {
+        const inlineHtml = super.render(imageUrl);
+        return DOMPurify.sanitize(`<div>${inlineHtml}</div>`);
+    }
+}
+
+export default BlockImageProcessor;
--- a/backend/src/services/learning-objects/processing/image/inline-image-processor.ts
+++ b/backend/src/services/learning-objects/processing/image/inline-image-processor.ts
@ -0,0 +1,24 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/image/inline_image_processor.js
+ */
+
+import DOMPurify from 'isomorphic-dompurify';
+import { DwengoContentType } from '../content-type.js';
+import { ProcessingError } from '../processing-error.js';
+import { isValidHttpUrl } from '../../../../util/links.js';
+import { StringProcessor } from '../string-processor.js';
+
+class InlineImageProcessor extends StringProcessor {
+    constructor(contentType: DwengoContentType = DwengoContentType.IMAGE_INLINE) {
+        super(contentType);
+    }
+
+    override renderFn(imageUrl: string) {
+        if (!isValidHttpUrl(imageUrl)) {
+            throw new ProcessingError(`Image URL is invalid: ${imageUrl}`);
+        }
+        return DOMPurify.sanitize(`<img src="${imageUrl}" alt="">`);
+    }
+}
+
+export default InlineImageProcessor;
--- a/backend/src/services/learning-objects/processing/markdown/dwengo-marked-renderer.ts
+++ b/backend/src/services/learning-objects/processing/markdown/dwengo-marked-renderer.ts
@ -0,0 +1,109 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/markdown/learing_object_markdown_renderer.js [sic!]
+ */
+import PdfProcessor from '../pdf/pdf-processor.js';
+import AudioProcessor from '../audio/audio-processor.js';
+import ExternProcessor from '../extern/extern-processor.js';
+import InlineImageProcessor from '../image/inline-image-processor.js';
+import * as marked from 'marked';
+import { getUrlStringForLearningObjectHTML, isValidHttpUrl } from '../../../../util/links.js';
+import { ProcessingError } from '../processing-error.js';
+import { LearningObjectIdentifier } from '../../../../interfaces/learning-content.js';
+import { Language } from '../../../../entities/content/language.js';
+
+import Image = marked.Tokens.Image;
+import Heading = marked.Tokens.Heading;
+import Link = marked.Tokens.Link;
+import RendererObject = marked.RendererObject;
+
+const prefixes = {
+    learningObject: '@learning-object',
+    pdf: '@pdf',
+    audio: '@audio',
+    extern: '@extern',
+    video: '@youtube',
+    notebook: '@notebook',
+    blockly: '@blockly',
+};
+
+function extractLearningObjectIdFromHref(href: string): LearningObjectIdentifier {
+    const [hruid, language, version] = href.split(/\/(.+)/, 2)[1].split('/');
+    return {
+        hruid,
+        language: language as Language,
+        version: parseInt(version),
+    };
+}
+
+/**
+ * An extension for the renderer of the Marked Markdown renderer which adds support for
+ * - a custom heading,
+ * - links to other learning objects,
+ * - embeddings of other learning objects.
+ */
+const dwengoMarkedRenderer: RendererObject = {
+    heading(heading: Heading): string {
+        const text = heading.text;
+        const level = heading.depth;
+        const escapedText = text.toLowerCase().replace(/[^\w]+/g, '-');
+
+        return (
+            `<h${level}>\n` +
+            `    <a name="${escapedText}" class="anchor" href="#${escapedText}">\n` +
+            `        <span class="header-link"></span>\n` +
+            `    </a>\n` +
+            `    ${text}\n` +
+            `</h${level}>\n`
+        );
+    },
+
+    // When the syntax for a link is used => [text](href "title")
+    // Render a custom link when the prefix for a learning object is used.
+    link(link: Link): string {
+        const href = link.href;
+        const title = link.title || '';
+        const text = marked.parseInline(link.text); // There could for example be an image in the link.
+
+        if (href.startsWith(prefixes.learningObject)) {
+            // Link to learning-object
+            const learningObjectId = extractLearningObjectIdFromHref(href);
+            return `<a href="${getUrlStringForLearningObjectHTML(learningObjectId)}" target="_blank" title="${title}">${text}</a>`;
+        }
+        // Any other link
+        if (!isValidHttpUrl(href)) {
+            throw new ProcessingError('Link is not a valid HTTP URL!');
+        }
+        //<a href="https://kiks.ilabt.imec.be/hub/tmplogin?id=0101" title="Notebooks Werking"><img src="Knop.png" alt="" title="Knop"></a>
+        return `<a href="${href}" target="_blank" title="${title}">${text}</a>`;
+    },
+
+    // When the syntax for an image is used => ![text](href "title")
+    // Render a learning object, pdf, audio or video if a prefix is used.
+    image(img: Image): string {
+        const href = img.href;
+        if (href.startsWith(prefixes.learningObject)) {
+            // Embedded learning-object
+            const learningObjectId = extractLearningObjectIdFromHref(href);
+            return `
+                <learning-object hruid="${learningObjectId.hruid}" language="${learningObjectId.language}" version="${learningObjectId.version}"/>
+            `; // Placeholder for the learning object since we cannot fetch its HTML here (this has to be a sync function!)
+        } else if (href.startsWith(prefixes.pdf)) {
+            // Embedded pdf
+            const proc = new PdfProcessor();
+            return proc.render(href.split(/\/(.+)/, 2)[1]);
+        } else if (href.startsWith(prefixes.audio)) {
+            // Embedded audio
+            const proc = new AudioProcessor();
+            return proc.render(href.split(/\/(.+)/, 2)[1]);
+        } else if (href.startsWith(prefixes.extern) || href.startsWith(prefixes.video) || href.startsWith(prefixes.notebook)) {
+            // Embedded youtube video or notebook (or other extern content)
+            const proc = new ExternProcessor();
+            return proc.render(href.split(/\/(.+)/, 2)[1]);
+        }
+        // Embedded image
+        const proc = new InlineImageProcessor();
+        return proc.render(href);
+    },
+};
+
+export default dwengoMarkedRenderer;
--- a/backend/src/services/learning-objects/processing/markdown/markdown-processor.ts
+++ b/backend/src/services/learning-objects/processing/markdown/markdown-processor.ts
@ -0,0 +1,39 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/markdown/markdown_processor.js
+ */
+
+import { marked } from 'marked';
+import InlineImageProcessor from '../image/inline-image-processor.js';
+import { DwengoContentType } from '../content-type.js';
+import dwengoMarkedRenderer from './dwengo-marked-renderer.js';
+import { StringProcessor } from '../string-processor.js';
+import { ProcessingError } from '../processing-error.js';
+
+class MarkdownProcessor extends StringProcessor {
+    constructor() {
+        super(DwengoContentType.TEXT_MARKDOWN);
+    }
+
+    override renderFn(mdText: string) {
+        let html = '';
+        try {
+            marked.use({ renderer: dwengoMarkedRenderer });
+            html = marked(mdText, { async: false });
+            html = this.replaceLinks(html); // Replace html image links path
+        } catch (e: any) {
+            throw new ProcessingError(e.message);
+        }
+        return html;
+    }
+
+    replaceLinks(html: string) {
+        const proc = new InlineImageProcessor();
+        html = html.replace(
+            /<img.*?src="(.*?)".*?(alt="(.*?)")?.*?(title="(.*?)")?.*?>/g,
+            (match: string, src: string, alt: string, altText: string, title: string, titleText: string) => proc.render(src)
+        );
+        return html;
+    }
+}
+
+export { MarkdownProcessor };
--- a/backend/src/services/learning-objects/processing/pdf/pdf-processor.ts
+++ b/backend/src/services/learning-objects/processing/pdf/pdf-processor.ts
@ -0,0 +1,32 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/pdf/pdf_processor.js
+ *
+ * WARNING: The support for PDF learning objects is currently still experimental.
+ */
+
+import DOMPurify from 'isomorphic-dompurify';
+import { DwengoContentType } from '../content-type.js';
+import { isValidHttpUrl } from '../../../../util/links.js';
+import { ProcessingError } from '../processing-error.js';
+import { StringProcessor } from '../string-processor.js';
+
+class PdfProcessor extends StringProcessor {
+    constructor() {
+        super(DwengoContentType.APPLICATION_PDF);
+    }
+
+    override renderFn(pdfUrl: string) {
+        if (!isValidHttpUrl(pdfUrl)) {
+            throw new ProcessingError(`PDF URL is invalid: ${pdfUrl}`);
+        }
+
+        return DOMPurify.sanitize(
+            `
+            <embed src="${pdfUrl}" type="application/pdf" width="100%" height="800px"/>
+            `,
+            { ADD_TAGS: ['embed'] }
+        );
+    }
+}
+
+export default PdfProcessor;
--- a/backend/src/services/learning-objects/processing/processing-error.ts
+++ b/backend/src/services/learning-objects/processing/processing-error.ts
@ -0,0 +1,5 @@
+export class ProcessingError extends Error {
+    constructor(error: string) {
+        super(error);
+    }
+}
--- a/backend/src/services/learning-objects/processing/processing-service.ts
+++ b/backend/src/services/learning-objects/processing/processing-service.ts
@ -0,0 +1,83 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/processing_proxy.js
+ */
+
+import BlockImageProcessor from './image/block-image-processor.js';
+import InlineImageProcessor from './image/inline-image-processor.js';
+import { MarkdownProcessor } from './markdown/markdown-processor.js';
+import TextProcessor from './text/text-processor.js';
+import AudioProcessor from './audio/audio-processor.js';
+import PdfProcessor from './pdf/pdf-processor.js';
+import ExternProcessor from './extern/extern-processor.js';
+import GiftProcessor from './gift/gift-processor.js';
+import { LearningObject } from '../../../entities/content/learning-object.entity.js';
+import Processor from './processor.js';
+import { DwengoContentType } from './content-type.js';
+import { LearningObjectIdentifier } from '../../../interfaces/learning-content.js';
+import { Language } from '../../../entities/content/language.js';
+import { replaceAsync } from '../../../util/async.js';
+
+const EMBEDDED_LEARNING_OBJECT_PLACEHOLDER = /<learning-object hruid="([^"]+)" language="([^"]+)" version="([^"]+)"\/>/g;
+const LEARNING_OBJECT_DOES_NOT_EXIST = "<div class='non-existing-learning-object' />";
+
+class ProcessingService {
+    private processors!: Map<DwengoContentType, Processor<any>>;
+
+    constructor() {
+        const processors = [
+            new InlineImageProcessor(),
+            new BlockImageProcessor(),
+            new MarkdownProcessor(),
+            new TextProcessor(),
+            new AudioProcessor(),
+            new PdfProcessor(),
+            new ExternProcessor(),
+            new GiftProcessor(),
+        ];
+
+        this.processors = new Map(processors.map((processor) => [processor.contentType, processor]));
+    }
+
+    /**
+     * Render the given learning object.
+     * @param learningObject The learning object to render
+     * @param fetchEmbeddedLearningObjects A function which takes a learning object identifier as an argument and
+     *                                     returns the corresponding learning object. This is used to fetch learning
+     *                                     objects embedded into this one.
+     *                                     If this argument is omitted, embedded learning objects will be represented
+     *                                     by placeholders.
+     * @returns Rendered HTML for this LearningObject as a string.
+     */
+    async render(
+        learningObject: LearningObject,
+        fetchEmbeddedLearningObjects?: (loId: LearningObjectIdentifier) => Promise<LearningObject | null>
+    ): Promise<string> {
+        const html = this.processors.get(learningObject.contentType)!.renderLearningObject(learningObject);
+        if (fetchEmbeddedLearningObjects) {
+            // Replace all embedded learning objects.
+            return replaceAsync(
+                html,
+                EMBEDDED_LEARNING_OBJECT_PLACEHOLDER,
+                async (_, hruid: string, language: string, version: string): Promise<string> => {
+                    // Fetch the embedded learning object...
+                    const learningObject = await fetchEmbeddedLearningObjects({
+                        hruid,
+                        language: language as Language,
+                        version: parseInt(version),
+                    });
+
+                    // If it does not exist, replace it by a placeholder.
+                    if (!learningObject) {
+                        return LEARNING_OBJECT_DOES_NOT_EXIST;
+                    }
+
+                    // ... and render it.
+                    return this.render(learningObject);
+                }
+            );
+        }
+        return html;
+    }
+}
+
+export default new ProcessingService();
--- a/backend/src/services/learning-objects/processing/processor.ts
+++ b/backend/src/services/learning-objects/processing/processor.ts
@ -0,0 +1,61 @@
+import { LearningObject } from '../../../entities/content/learning-object.entity.js';
+import { ProcessingError } from './processing-error.js';
+import { DwengoContentType } from './content-type.js';
+
+/**
+ * Abstract base class for all processors.
+ * Each processor is responsible for a specific format a learning object can be in, which i tcan render to HTML.
+ *
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/processor.js
+ */
+abstract class Processor<T> {
+    protected constructor(public contentType: DwengoContentType) {}
+
+    /**
+     * Render the given object.
+     *
+     * @param toRender Object which has to be rendered to HTML. This object has to be in the format for which this
+     *                 Processor is responsible.
+     * @return Rendered HTML-string
+     * @throws ProcessingError if the rendering fails.
+     */
+    render(toRender: T): string {
+        return this.renderFn(toRender);
+    }
+
+    /**
+     * Render a learning object with the content type for which this processor is responsible.
+     * @param toRender
+     */
+    renderLearningObject(toRender: LearningObject): string {
+        if (toRender.contentType !== this.contentType) {
+            throw new ProcessingError(
+                `Unsupported content type: ${toRender.contentType}.
+                This processor is only responsible for content of type ${this.contentType}.`
+            );
+        }
+        return this.renderLearningObjectFn(toRender);
+    }
+
+    /**
+     * Function which actually renders the content.
+     *
+     * @param toRender Content to be rendered
+     * @return Rendered HTML as a string
+     * @protected
+     */
+    protected abstract renderFn(toRender: T): string;
+
+    /**
+     * Function which actually executes the rendering of a learning object.
+     *
+     * When implementing this function, we may assume that we are responsible for the content type of the learning
+     * object.
+     *
+     * @param toRender Learning object to render
+     * @protected
+     */
+    protected abstract renderLearningObjectFn(toRender: LearningObject): string;
+}
+
+export default Processor;
--- a/backend/src/services/learning-objects/processing/string-processor.ts
+++ b/backend/src/services/learning-objects/processing/string-processor.ts
@ -0,0 +1,19 @@
+import Processor from './processor.js';
+import { LearningObject } from '../../../entities/content/learning-object.entity.js';
+
+export abstract class StringProcessor extends Processor<string> {
+    /**
+     * Function which actually executes the rendering of a learning object.
+     * By default, this just means rendering the content in the content property of the learning object (interpreted
+     * as string)
+     *
+     * When implementing this function, we may assume that we are responsible for the content type of the learning
+     * object.
+     *
+     * @param toRender Learning object to render
+     * @protected
+     */
+    protected renderLearningObjectFn(toRender: LearningObject): string {
+        return this.render(toRender.content.toString('ascii'));
+    }
+}
--- a/backend/src/services/learning-objects/processing/text/text-processor.ts
+++ b/backend/src/services/learning-objects/processing/text/text-processor.ts
@ -0,0 +1,20 @@
+/**
+ * Based on https://github.com/dwengovzw/Learning-Object-Repository/blob/main/app/processors/text/text_processor.js
+ */
+
+import DOMPurify from 'isomorphic-dompurify';
+import { DwengoContentType } from '../content-type.js';
+import { StringProcessor } from '../string-processor.js';
+
+class TextProcessor extends StringProcessor {
+    constructor() {
+        super(DwengoContentType.TEXT_PLAIN);
+    }
+
+    override renderFn(text: string) {
+        // Sanitize plain text to prevent xss.
+        return DOMPurify.sanitize(text);
+    }
+}
+
+export default TextProcessor;
--- a/backend/src/services/learning-paths/database-learning-path-provider.ts
+++ b/backend/src/services/learning-paths/database-learning-path-provider.ts
@ -0,0 +1,190 @@
+import { LearningPathProvider } from './learning-path-provider.js';
+import { FilteredLearningObject, LearningObjectNode, LearningPath, LearningPathResponse, Transition } from '../../interfaces/learning-content.js';
+import { LearningPath as LearningPathEntity } from '../../entities/content/learning-path.entity.js';
+import { getLearningPathRepository } from '../../data/repositories.js';
+import { Language } from '../../entities/content/language.js';
+import learningObjectService from '../learning-objects/learning-object-service.js';
+import { LearningPathNode } from '../../entities/content/learning-path-node.entity.js';
+import { LearningPathTransition } from '../../entities/content/learning-path-transition.entity.js';
+import { getLastSubmissionForCustomizationTarget, isTransitionPossible, PersonalizationTarget } from './learning-path-personalization-util.js';
+
+/**
+ * Fetches the corresponding learning object for each of the nodes and creates a map that maps each node to its
+ * corresponding learning object.
+ * @param nodes The nodes to find the learning object for.
+ */
+async function getLearningObjectsForNodes(nodes: LearningPathNode[]): Promise<Map<LearningPathNode, FilteredLearningObject>> {
+    // Fetching the corresponding learning object for each of the nodes and creating a map that maps each node to
+    // Its corresponding learning object.
+    const nullableNodesToLearningObjects = new Map<LearningPathNode, FilteredLearningObject | null>(
+        await Promise.all(
+            nodes.map((node) =>
+                learningObjectService
+                    .getLearningObjectById({
+                        hruid: node.learningObjectHruid,
+                        version: node.version,
+                        language: node.language,
+                    })
+                    .then((learningObject) => <[LearningPathNode, FilteredLearningObject | null]>[node, learningObject])
+            )
+        )
+    );
+    if (nullableNodesToLearningObjects.values().some((it) => it === null)) {
+        throw new Error('At least one of the learning objects on this path could not be found.');
+    }
+    return nullableNodesToLearningObjects as Map<LearningPathNode, FilteredLearningObject>;
+}
+
+/**
+ * Convert the given learning path entity to an object which conforms to the learning path content.
+ */
+async function convertLearningPath(learningPath: LearningPathEntity, order: number, personalizedFor?: PersonalizationTarget): Promise<LearningPath> {
+    const nodesToLearningObjects: Map<LearningPathNode, FilteredLearningObject> = await getLearningObjectsForNodes(learningPath.nodes);
+
+    const targetAges = nodesToLearningObjects
+        .values()
+        .flatMap((it) => it.targetAges || [])
+        .toArray();
+
+    const keywords = nodesToLearningObjects
+        .values()
+        .flatMap((it) => it.keywords || [])
+        .toArray();
+
+    const image = learningPath.image ? learningPath.image.toString('base64') : undefined;
+
+    const convertedNodes = await convertNodes(nodesToLearningObjects, personalizedFor);
+
+    return {
+        _id: `${learningPath.hruid}/${learningPath.language}`, // For backwards compatibility with the original Dwengo API.
+        __order: order,
+        hruid: learningPath.hruid,
+        language: learningPath.language,
+        description: learningPath.description,
+        image: image,
+        title: learningPath.title,
+        nodes: convertedNodes,
+        num_nodes: learningPath.nodes.length,
+        num_nodes_left: convertedNodes.filter((it) => !it.done).length,
+        keywords: keywords.join(' '),
+        target_ages: targetAges,
+        max_age: Math.max(...targetAges),
+        min_age: Math.min(...targetAges),
+    };
+}
+
+/**
+ * Helper function converting pairs of learning path nodes (as represented in the database) and the corresponding
+ * learning objects into a list of learning path nodes as they should be represented in the API.
+ * @param nodesToLearningObjects
+ * @param personalizedFor
+ */
+async function convertNodes(
+    nodesToLearningObjects: Map<LearningPathNode, FilteredLearningObject>,
+    personalizedFor?: PersonalizationTarget
+): Promise<LearningObjectNode[]> {
+    const nodesPromise = nodesToLearningObjects
+        .entries()
+        .map(async (entry) => {
+            const [node, learningObject] = entry;
+            const lastSubmission = personalizedFor ? await getLastSubmissionForCustomizationTarget(node, personalizedFor) : null;
+            return {
+                _id: learningObject.uuid,
+                language: learningObject.language,
+                start_node: node.startNode,
+                created_at: node.createdAt.toISOString(),
+                updatedAt: node.updatedAt.toISOString(),
+                learningobject_hruid: node.learningObjectHruid,
+                version: learningObject.version,
+                transitions: node.transitions
+                    .filter(
+                        (trans) => !personalizedFor || isTransitionPossible(trans, optionalJsonStringToObject(lastSubmission?.content)) // If we want a personalized learning path, remove all transitions that aren't possible.
+                    )
+                    .map((trans, i) => convertTransition(trans, i, nodesToLearningObjects)), // Then convert all the transition
+            };
+        })
+        .toArray();
+    return await Promise.all(nodesPromise);
+}
+
+/**
+ * Helper method to convert a json string to an object, or null if it is undefined.
+ */
+function optionalJsonStringToObject(jsonString?: string): object | null {
+    if (!jsonString) {
+        return null;
+    }
+    return JSON.parse(jsonString);
+}
+
+/**
+ * Helper function which converts a transition in the database representation to a transition in the representation
+ * the Dwengo API uses.
+ *
+ * @param transition
+ * @param index
+ * @param nodesToLearningObjects
+ */
+function convertTransition(
+    transition: LearningPathTransition,
+    index: number,
+    nodesToLearningObjects: Map<LearningPathNode, FilteredLearningObject>
+): Transition {
+    const nextNode = nodesToLearningObjects.get(transition.next);
+    if (!nextNode) {
+        throw new Error(`Learning object ${transition.next.learningObjectHruid}/${transition.next.language}/${transition.next.version} not found!`);
+    } else {
+        return {
+            _id: '' + index, // Retained for backwards compatibility. The index uniquely identifies the transition within the learning path.
+            default: false, // We don't work with default transitions but retain this for backwards compatibility.
+            next: {
+                _id: nextNode._id + index, // Construct a unique ID for the transition for backwards compatibility.
+                hruid: transition.next.learningObjectHruid,
+                language: nextNode.language,
+                version: nextNode.version,
+            },
+        };
+    }
+}
+
+/**
+ * Service providing access to data about learning paths from the database.
+ */
+const databaseLearningPathProvider: LearningPathProvider = {
+    /**
+     * Fetch the learning paths with the given hruids from the database.
+     */
+    async fetchLearningPaths(
+        hruids: string[],
+        language: Language,
+        source: string,
+        personalizedFor?: PersonalizationTarget
+    ): Promise<LearningPathResponse> {
+        const learningPathRepo = getLearningPathRepository();
+
+        const learningPaths = (await Promise.all(hruids.map((hruid) => learningPathRepo.findByHruidAndLanguage(hruid, language)))).filter(
+            (learningPath) => learningPath !== null
+        );
+        const filteredLearningPaths = await Promise.all(
+            learningPaths.map((learningPath, index) => convertLearningPath(learningPath, index, personalizedFor))
+        );
+
+        return {
+            success: filteredLearningPaths.length > 0,
+            data: await Promise.all(filteredLearningPaths),
+            source,
+        };
+    },
+
+    /**
+     * Search learning paths in the database using the given search string.
+     */
+    async searchLearningPaths(query: string, language: Language, personalizedFor?: PersonalizationTarget): Promise<LearningPath[]> {
+        const learningPathRepo = getLearningPathRepository();
+
+        const searchResults = await learningPathRepo.findByQueryStringAndLanguage(query, language);
+        return await Promise.all(searchResults.map((result, index) => convertLearningPath(result, index, personalizedFor)));
+    },
+};
+
+export default databaseLearningPathProvider;
--- a/backend/src/services/learning-paths/dwengo-api-learning-path-provider.ts
+++ b/backend/src/services/learning-paths/dwengo-api-learning-path-provider.ts
@ -0,0 +1,50 @@
+import { fetchWithLogging } from '../../util/apiHelper.js';
+import { DWENGO_API_BASE } from '../../config.js';
+import { LearningPath, LearningPathResponse } from '../../interfaces/learning-content.js';
+import { LearningPathProvider } from './learning-path-provider.js';
+import { getLogger, Logger } from '../../logging/initalize.js';
+
+const logger: Logger = getLogger();
+
+const dwengoApiLearningPathProvider: LearningPathProvider = {
+    async fetchLearningPaths(hruids: string[], language: string, source: string): Promise<LearningPathResponse> {
+        if (hruids.length === 0) {
+            return {
+                success: false,
+                source,
+                data: null,
+                message: `No HRUIDs provided for ${source}.`,
+            };
+        }
+
+        const apiUrl = `${DWENGO_API_BASE}/learningPath/getPathsFromIdList`;
+        const params = { pathIdList: JSON.stringify({ hruids }), language };
+
+        const learningPaths = await fetchWithLogging<LearningPath[]>(apiUrl, `Learning paths for ${source}`, { params });
+
+        if (!learningPaths || learningPaths.length === 0) {
+            logger.warn(`⚠️ WARNING: No learning paths found for ${source}.`);
+            return {
+                success: false,
+                source,
+                data: [],
+                message: `No learning paths found for ${source}.`,
+            };
+        }
+
+        return {
+            success: true,
+            source,
+            data: learningPaths,
+        };
+    },
+    async searchLearningPaths(query: string, language: string): Promise<LearningPath[]> {
+        const apiUrl = `${DWENGO_API_BASE}/learningPath/search`;
+        const params = { all: query, language };
+
+        const searchResults = await fetchWithLogging<LearningPath[]>(apiUrl, `Search learning paths with query "${query}"`, { params });
+        return searchResults ?? [];
+    },
+};
+
+export default dwengoApiLearningPathProvider;
--- a/backend/src/services/learning-paths/learning-path-personalization-util.ts
+++ b/backend/src/services/learning-paths/learning-path-personalization-util.ts
@ -0,0 +1,90 @@
+import { LearningPathNode } from '../../entities/content/learning-path-node.entity.js';
+import { Student } from '../../entities/users/student.entity.js';
+import { Group } from '../../entities/assignments/group.entity.js';
+import { Submission } from '../../entities/assignments/submission.entity.js';
+import { getClassRepository, getGroupRepository, getStudentRepository, getSubmissionRepository } from '../../data/repositories.js';
+import { LearningObjectIdentifier } from '../../entities/content/learning-object-identifier.js';
+import { LearningPathTransition } from '../../entities/content/learning-path-transition.entity.js';
+import { JSONPath } from 'jsonpath-plus';
+
+export type PersonalizationTarget = { type: 'student'; student: Student } | { type: 'group'; group: Group };
+
+/**
+ * Shortcut function to easily create a PersonalizationTarget object for a student by his/her username.
+ * @param username Username of the student we want to generate a personalized learning path for.
+ *                 If there is no student with this username, return undefined.
+ */
+export async function personalizedForStudent(username: string): Promise<PersonalizationTarget | undefined> {
+    const student = await getStudentRepository().findByUsername(username);
+    if (student) {
+        return {
+            type: 'student',
+            student: student,
+        };
+    }
+    return undefined;
+}
+
+/**
+ * Shortcut function to easily create a PersonalizationTarget object for a group by class name, assignment number and
+ * group number.
+ * @param classId Id of the class in which this group was created
+ * @param assignmentNumber Number of the assignment for which this group was created
+ * @param groupNumber Number of the group for which we want to personalize the learning path.
+ */
+export async function personalizedForGroup(
+    classId: string,
+    assignmentNumber: number,
+    groupNumber: number
+): Promise<PersonalizationTarget | undefined> {
+    const clazz = await getClassRepository().findById(classId);
+    if (!clazz) {
+        return undefined;
+    }
+    const group = await getGroupRepository().findOne({
+        assignment: {
+            within: clazz,
+            id: assignmentNumber,
+        },
+        groupNumber: groupNumber,
+    });
+    if (group) {
+        return {
+            type: 'group',
+            group: group,
+        };
+    }
+    return undefined;
+}
+
+/**
+ * Returns the last submission for the learning object associated with the given node and for the student or group
+ */
+export async function getLastSubmissionForCustomizationTarget(node: LearningPathNode, pathFor: PersonalizationTarget): Promise<Submission | null> {
+    const submissionRepo = getSubmissionRepository();
+    const learningObjectId: LearningObjectIdentifier = {
+        hruid: node.learningObjectHruid,
+        language: node.language,
+        version: node.version,
+    };
+    if (pathFor.type === 'group') {
+        return await submissionRepo.findMostRecentSubmissionForGroup(learningObjectId, pathFor.group);
+    }
+    return await submissionRepo.findMostRecentSubmissionForStudent(learningObjectId, pathFor.student);
+}
+
+/**
+ * Checks whether the condition of the given transaction is fulfilled by the given submission.
+ * @param transition
+ * @param submitted
+ */
+export function isTransitionPossible(transition: LearningPathTransition, submitted: object | null): boolean {
+    if (transition.condition === 'true' || !transition.condition) {
+        return true; // If the transition is unconditional, we can go on.
+    }
+    if (submitted === null) {
+        return false; // If the transition is not unconditional and there was no submission, the transition is not possible.
+    }
+    const match = JSONPath({ path: transition.condition, json: { submission: submitted } });
+    return match.length === 1;
+}
--- a/backend/src/services/learning-paths/learning-path-provider.ts
+++ b/backend/src/services/learning-paths/learning-path-provider.ts
@ -0,0 +1,18 @@
+import { LearningPath, LearningPathResponse } from '../../interfaces/learning-content.js';
+import { Language } from '../../entities/content/language.js';
+import { PersonalizationTarget } from './learning-path-personalization-util.js';
+
+/**
+ * Generic interface for a service which provides access to learning paths from a data source.
+ */
+export interface LearningPathProvider {
+    /**
+     * Fetch the learning paths with the given hruids from the data source.
+     */
+    fetchLearningPaths(hruids: string[], language: Language, source: string, personalizedFor?: PersonalizationTarget): Promise<LearningPathResponse>;
+
+    /**
+     * Search learning paths in the data source using the given search string.
+     */
+    searchLearningPaths(query: string, language: Language, personalizedFor?: PersonalizationTarget): Promise<LearningPath[]>;
+}
--- a/backend/src/services/learning-paths/learning-path-service.ts
+++ b/backend/src/services/learning-paths/learning-path-service.ts
@ -0,0 +1,57 @@
+import { LearningPath, LearningPathResponse } from '../../interfaces/learning-content.js';
+import dwengoApiLearningPathProvider from './dwengo-api-learning-path-provider.js';
+import databaseLearningPathProvider from './database-learning-path-provider.js';
+import { EnvVars, getEnvVar } from '../../util/envvars.js';
+import { Language } from '../../entities/content/language.js';
+import { PersonalizationTarget } from './learning-path-personalization-util.js';
+
+const userContentPrefix = getEnvVar(EnvVars.UserContentPrefix);
+const allProviders = [dwengoApiLearningPathProvider, databaseLearningPathProvider];
+
+/**
+ * Service providing access to data about learning paths from the appropriate data source (database or Dwengo-api)
+ */
+const learningPathService = {
+    /**
+     * Fetch the learning paths with the given hruids from the data source.
+     * @param hruids For each of the hruids, the learning path will be fetched.
+     * @param language This is the language each of the learning paths will use.
+     * @param source
+     * @param personalizedFor If this is set, a learning path personalized for the given group or student will be returned.
+     */
+    async fetchLearningPaths(
+        hruids: string[],
+        language: Language,
+        source: string,
+        personalizedFor?: PersonalizationTarget
+    ): Promise<LearningPathResponse> {
+        const userContentHruids = hruids.filter((hruid) => hruid.startsWith(userContentPrefix));
+        const nonUserContentHruids = hruids.filter((hruid) => !hruid.startsWith(userContentPrefix));
+
+        const userContentLearningPaths = await databaseLearningPathProvider.fetchLearningPaths(userContentHruids, language, source, personalizedFor);
+        const nonUserContentLearningPaths = await dwengoApiLearningPathProvider.fetchLearningPaths(
+            nonUserContentHruids,
+            language,
+            source,
+            personalizedFor
+        );
+
+        const result = (userContentLearningPaths.data || []).concat(nonUserContentLearningPaths.data || []);
+
+        return {
+            data: result,
+            source: source,
+            success: userContentLearningPaths.success || nonUserContentLearningPaths.success,
+        };
+    },
+
+    /**
+     * Search learning paths in the data source using the given search string.
+     */
+    async searchLearningPaths(query: string, language: Language, personalizedFor?: PersonalizationTarget): Promise<LearningPath[]> {
+        const providerResponses = await Promise.all(allProviders.map((provider) => provider.searchLearningPaths(query, language, personalizedFor)));
+        return providerResponses.flat();
+    },
+};
+
+export default learningPathService;
--- a/backend/src/services/learningObjects.ts
+++ b/backend/src/services/learningObjects.ts
@ -1,91 +0,0 @@
-import { DWENGO_API_BASE } from '../config.js';
-import { fetchWithLogging } from '../util/apiHelper.js';
-import { FilteredLearningObject, LearningObjectMetadata, LearningObjectNode, LearningPathResponse } from '../interfaces/learningPath.js';
-import { fetchLearningPaths } from './learningPaths.js';
-import { getLogger, Logger } from '../logging/initalize.js';
-
-const logger: Logger = getLogger();
-
-function filterData(data: LearningObjectMetadata, htmlUrl: string): FilteredLearningObject {
-    return {
-        key: data.hruid, // Hruid learningObject (not path)
-        _id: data._id,
-        uuid: data.uuid,
-        version: data.version,
-        title: data.title,
-        htmlUrl, // Url to fetch html content
-        language: data.language,
-        difficulty: data.difficulty,
-        estimatedTime: data.estimated_time,
-        available: data.available,
-        teacherExclusive: data.teacher_exclusive,
-        educationalGoals: data.educational_goals, // List with learningObjects
-        keywords: data.keywords, // For search
-        description: data.description, // For search (not an actual description)
-        targetAges: data.target_ages,
-        contentType: data.content_type, // Markdown, image, audio, etc.
-        contentLocation: data.content_location, // If content type extern
-        skosConcepts: data.skos_concepts,
-        returnValue: data.return_value, // Callback response information
-    };
-}
-
-/**
- * Fetches a single learning object by its HRUID
- */
-export async function getLearningObjectById(hruid: string, language: string): Promise<FilteredLearningObject | null> {
-    const metadataUrl = `${DWENGO_API_BASE}/learningObject/getMetadata?hruid=${hruid}&language=${language}`;
-    const metadata = await fetchWithLogging<LearningObjectMetadata>(
-        metadataUrl,
-        `Metadata for Learning Object HRUID "${hruid}" (language ${language})`
-    );
-
-    if (!metadata) {
-        logger.warn(`⚠️ WARNING: Learning object "${hruid}" not found.`);
-        return null;
-    }
-
-    const htmlUrl = `${DWENGO_API_BASE}/learningObject/getRaw?hruid=${hruid}&language=${language}`;
-    return filterData(metadata, htmlUrl);
-}
-
-/**
- * Generic function to fetch learning objects (full data or just HRUIDs)
- */
-async function fetchLearningObjects(hruid: string, full: boolean, language: string): Promise<FilteredLearningObject[] | string[]> {
-    try {
-        const learningPathResponse: LearningPathResponse = await fetchLearningPaths([hruid], language, `Learning path for HRUID "${hruid}"`);
-
-        if (!learningPathResponse.success || !learningPathResponse.data?.length) {
-            logger.warn(`⚠️ WARNING: Learning path "${hruid}" exists but contains no learning objects.`);
-            return [];
-        }
-
-        const nodes: LearningObjectNode[] = learningPathResponse.data[0].nodes;
-
-        if (!full) {
-            return nodes.map((node) => node.learningobject_hruid);
-        }
-
-        return await Promise.all(nodes.map(async (node) => getLearningObjectById(node.learningobject_hruid, language))).then((objects) =>
-            objects.filter((obj): obj is FilteredLearningObject => obj !== null)
-        );
-    } catch (error) {
-        logger.error('❌ Error fetching learning objects:', error);
-        return [];
-    }
-}
-
-/**
- * Fetch full learning object data (metadata)
- */
-export async function getLearningObjectsFromPath(hruid: string, language: string): Promise<FilteredLearningObject[]> {
-    return (await fetchLearningObjects(hruid, true, language)) as FilteredLearningObject[];
-}
-
-/**
- * Fetch only learning object HRUIDs
- */
-export async function getLearningObjectIdsFromPath(hruid: string, language: string): Promise<string[]> {
-    return (await fetchLearningObjects(hruid, false, language)) as string[];
-}
--- a/backend/src/services/learningPaths.ts
+++ b/backend/src/services/learningPaths.ts
@ -1,46 +0,0 @@
-import { fetchWithLogging } from '../util/apiHelper.js';
-import { DWENGO_API_BASE } from '../config.js';
-import { LearningPath, LearningPathResponse } from '../interfaces/learningPath.js';
-import { getLogger, Logger } from '../logging/initalize.js';
-
-const logger: Logger = getLogger();
-
-export async function fetchLearningPaths(hruids: string[], language: string, source: string): Promise<LearningPathResponse> {
-    if (hruids.length === 0) {
-        return {
-            success: false,
-            source,
-            data: null,
-            message: `No HRUIDs provided for ${source}.`,
-        };
-    }
-
-    const apiUrl = `${DWENGO_API_BASE}/learningPath/getPathsFromIdList`;
-    const params = { pathIdList: JSON.stringify({ hruids }), language };
-
-    const learningPaths = await fetchWithLogging<LearningPath[]>(apiUrl, `Learning paths for ${source}`, params);
-
-    if (!learningPaths || learningPaths.length === 0) {
-        logger.warn(`⚠️ WARNING: No learning paths found for ${source}.`);
-        return {
-            success: false,
-            source,
-            data: [],
-            message: `No learning paths found for ${source}.`,
-        };
-    }
-
-    return {
-        success: true,
-        source,
-        data: learningPaths,
-    };
-}
-
-export async function searchLearningPaths(query: string, language: string): Promise<LearningPath[]> {
-    const apiUrl = `${DWENGO_API_BASE}/learningPath/search`;
-    const params = { all: query, language };
-
-    const searchResults = await fetchWithLogging<LearningPath[]>(apiUrl, `Search learning paths with query "${query}"`, params);
-    return searchResults ?? [];
-}
--- a/backend/src/sqlite-autoincrement-workaround.ts
+++ b/backend/src/sqlite-autoincrement-workaround.ts
@ -0,0 +1,41 @@
+import { EntityProperty, EventArgs, EventSubscriber } from '@mikro-orm/core';
+
+/**
+ * The tests are ran on an in-memory SQLite database. However, SQLite does not allow fields which are part of composite
+ * primary keys to be autoincremented (while PostgreSQL, which we use in production, does). This Subscriber works around
+ * the issue by remembering the highest values for every autoincremented part of a primary key and assigning them when
+ * creating a new entity.
+ *
+ * However, it is important to note the following limitations:
+ * - this class can only be used for in-memory SQLite databases since the information on what the highest sequence
+ *   number for each of the properties is, is only saved transiently.
+ * - automatically setting the generated "autoincremented" value for properties only works when the entity is created
+ *   via an entityManager.create(...) or repo.create(...) method. Otherwise, onInit will not be called and therefore,
+ *   the sequence number will not be filled in.
+ */
+export class SqliteAutoincrementSubscriber implements EventSubscriber {
+    private sequenceNumbersForEntityType: Map<string, number> = new Map();
+
+    /**
+     * When an entity with an autoincremented property which is part of the composite private key is created,
+     * automatically fill this property so we won't face not-null-constraint exceptions when persisting it.
+     */
+    onInit<T extends object>(args: EventArgs<T>): void {
+        if (!args.meta.compositePK) {
+            return; // If there is not a composite primary key, autoincrement works fine with SQLite anyway.
+        }
+
+        for (const prop of Object.values(args.meta.properties)) {
+            const property = prop as EntityProperty<T>;
+            if (property.primary && property.autoincrement && !(args.entity as Record<string, any>)[property.name]) {
+                // Obtain and increment sequence number of this entity.
+                const propertyKey = args.meta.class.name + '.' + property.name;
+                const nextSeqNumber = this.sequenceNumbersForEntityType.get(propertyKey) || 0;
+                this.sequenceNumbersForEntityType.set(propertyKey, nextSeqNumber + 1);
+
+                // Set the property accordingly.
+                (args.entity as Record<string, any>)[property.name] = nextSeqNumber + 1;
+            }
+        }
+    }
+}
--- a/backend/src/util/apiHelper.ts
+++ b/backend/src/util/apiHelper.ts
@ -9,13 +9,21 @@ const logger: Logger = getLogger();
 *
 * @param url The API endpoint to fetch from.
 * @param description A short description of what is being fetched (for logging).
- * @param params
+ * @param options Contains further options such as params (the query params) and responseType (whether the response
+ *                should be parsed as JSON ("json") or whether it should be returned as plain text ("text")
 * @returns The response data if successful, or null if an error occurs.
 */
-export async function fetchWithLogging<T>(url: string, description: string, params?: Record<string, any>): Promise<T | null> {
+export async function fetchWithLogging<T>(
+    url: string,
+    description: string,
+    options?: {
+        params?: Record<string, any>;
+        query?: Record<string, any>;
+        responseType?: 'json' | 'text';
+    }
+): Promise<T | null> {
    try {
-        const config: AxiosRequestConfig = params ? { params } : {};
-
+        const config: AxiosRequestConfig = options || {};
        const response = await axios.get<T>(url, config);
        return response.data;
    } catch (error: any) {
--- a/backend/src/util/async.ts
+++ b/backend/src/util/async.ts
@ -0,0 +1,23 @@
+/**
+ * Replace all occurrences of regex in str with the result of asyncFn called with the matching snippet and each of
+ * the parts matched by a group in the regex as arguments.
+ *
+ * @param str The string where to replace the occurrences
+ * @param regex
+ * @param replacementFn
+ */
+export async function replaceAsync(str: string, regex: RegExp, replacementFn: (match: string, ...args: string[]) => Promise<string>) {
+    const promises: Promise<string>[] = [];
+
+    // First run through matches: add all Promises resulting from the replacement function
+    str.replace(regex, (full, ...args) => {
+        promises.push(replacementFn(full, ...args));
+        return full;
+    });
+
+    // Wait for the replacements to get loaded. Reverse them so when popping them, we work in a FIFO manner.
+    const replacements: string[] = await Promise.all(promises);
+
+    // Second run through matches: Replace them by their previously computed replacements.
+    return str.replace(regex, () => replacements.pop()!);
+}
--- a/backend/src/util/envvars.ts
+++ b/backend/src/util/envvars.ts
@ -15,6 +15,9 @@ export const EnvVars: { [key: string]: EnvVar } = {
    DbUsername: { key: DB_PREFIX + 'USERNAME', required: true },
    DbPassword: { key: DB_PREFIX + 'PASSWORD', required: true },
    DbUpdate: { key: DB_PREFIX + 'UPDATE', defaultValue: false },
+    LearningContentRepoApiBaseUrl: { key: PREFIX + 'LEARNING_CONTENT_REPO_API_BASE_URL', defaultValue: 'https://dwengo.org/backend/api' },
+    FallbackLanguage: { key: PREFIX + 'FALLBACK_LANGUAGE', defaultValue: 'nl' },
+    UserContentPrefix: { key: DB_PREFIX + 'USER_CONTENT_PREFIX', defaultValue: 'u_' },
    IdpStudentUrl: { key: STUDENT_IDP_PREFIX + 'URL', required: true },
    IdpStudentClientId: { key: STUDENT_IDP_PREFIX + 'CLIENT_ID', required: true },
    IdpStudentJwksEndpoint: { key: STUDENT_IDP_PREFIX + 'JWKS_ENDPOINT', required: true },
--- a/backend/src/util/links.ts
+++ b/backend/src/util/links.ts
@ -0,0 +1,26 @@
+import { LearningObjectIdentifier } from '../interfaces/learning-content';
+
+export function isValidHttpUrl(url: string): boolean {
+    try {
+        const parsedUrl = new URL(url, 'http://test.be');
+        return parsedUrl.protocol === 'http:' || parsedUrl.protocol === 'https:';
+    } catch (e) {
+        return false;
+    }
+}
+
+export function getUrlStringForLearningObject(learningObjectId: LearningObjectIdentifier) {
+    let url = `/learningObject/${learningObjectId.hruid}/html?language=${learningObjectId.language}`;
+    if (learningObjectId.version) {
+        url += `&version=${learningObjectId.version}`;
+    }
+    return url;
+}
+
+export function getUrlStringForLearningObjectHTML(learningObjectIdentifier: LearningObjectIdentifier): string {
+    let url = `/learningObject/${learningObjectIdentifier.hruid}/html?language=${learningObjectIdentifier.language}`;
+    if (learningObjectIdentifier.version) {
+        url += `&version=${learningObjectIdentifier.version}`;
+    }
+    return url;
+}
--- a/backend/tests/data/assignments/assignments.test.ts
+++ b/backend/tests/data/assignments/assignments.test.ts
@ -0,0 +1,42 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests';
+import { AssignmentRepository } from '../../../src/data/assignments/assignment-repository';
+import { getAssignmentRepository, getClassRepository } from '../../../src/data/repositories';
+import { ClassRepository } from '../../../src/data/classes/class-repository';
+
+describe('AssignmentRepository', () => {
+    let assignmentRepository: AssignmentRepository;
+    let classRepository: ClassRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        assignmentRepository = getAssignmentRepository();
+        classRepository = getClassRepository();
+    });
+
+    it('should return the requested assignment', async () => {
+        const class_ = await classRepository.findById('id02');
+        const assignment = await assignmentRepository.findByClassAndId(class_!, 2);
+
+        expect(assignment).toBeTruthy();
+        expect(assignment!.title).toBe('tool');
+    });
+
+    it('should return all assignments for a class', async () => {
+        const class_ = await classRepository.findById('id02');
+        const assignments = await assignmentRepository.findAllAssignmentsInClass(class_!);
+
+        expect(assignments).toBeTruthy();
+        expect(assignments).toHaveLength(1);
+        expect(assignments[0].title).toBe('tool');
+    });
+
+    it('should not find removed assignment', async () => {
+        const class_ = await classRepository.findById('id01');
+        await assignmentRepository.deleteByClassAndId(class_!, 3);
+
+        const assignment = await assignmentRepository.findByClassAndId(class_!, 3);
+
+        expect(assignment).toBeNull();
+    });
+});
--- a/backend/tests/data/assignments/groups.test.ts
+++ b/backend/tests/data/assignments/groups.test.ts
@ -0,0 +1,49 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests';
+import { GroupRepository } from '../../../src/data/assignments/group-repository';
+import { getAssignmentRepository, getClassRepository, getGroupRepository } from '../../../src/data/repositories';
+import { AssignmentRepository } from '../../../src/data/assignments/assignment-repository';
+import { ClassRepository } from '../../../src/data/classes/class-repository';
+
+describe('GroupRepository', () => {
+    let groupRepository: GroupRepository;
+    let assignmentRepository: AssignmentRepository;
+    let classRepository: ClassRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        groupRepository = getGroupRepository();
+        assignmentRepository = getAssignmentRepository();
+        classRepository = getClassRepository();
+    });
+
+    it('should return the requested group', async () => {
+        const class_ = await classRepository.findById('id01');
+        const assignment = await assignmentRepository.findByClassAndId(class_!, 1);
+
+        const group = await groupRepository.findByAssignmentAndGroupNumber(assignment!, 1);
+
+        expect(group).toBeTruthy();
+    });
+
+    it('should return all groups for assignment', async () => {
+        const class_ = await classRepository.findById('id01');
+        const assignment = await assignmentRepository.findByClassAndId(class_!, 1);
+
+        const groups = await groupRepository.findAllGroupsForAssignment(assignment!);
+
+        expect(groups).toBeTruthy();
+        expect(groups).toHaveLength(3);
+    });
+
+    it('should not find removed group', async () => {
+        const class_ = await classRepository.findById('id02');
+        const assignment = await assignmentRepository.findByClassAndId(class_!, 2);
+
+        await groupRepository.deleteByAssignmentAndGroupNumber(assignment!, 1);
+
+        const group = await groupRepository.findByAssignmentAndGroupNumber(assignment!, 1);
+
+        expect(group).toBeNull();
+    });
+});
--- a/backend/tests/data/assignments/submissions.test.ts
+++ b/backend/tests/data/assignments/submissions.test.ts
@ -0,0 +1,70 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests';
+import { SubmissionRepository } from '../../../src/data/assignments/submission-repository';
+import {
+    getAssignmentRepository,
+    getClassRepository,
+    getGroupRepository,
+    getStudentRepository,
+    getSubmissionRepository,
+} from '../../../src/data/repositories';
+import { LearningObjectIdentifier } from '../../../src/entities/content/learning-object-identifier';
+import { Language } from '../../../src/entities/content/language';
+import { StudentRepository } from '../../../src/data/users/student-repository';
+import { GroupRepository } from '../../../src/data/assignments/group-repository';
+import { AssignmentRepository } from '../../../src/data/assignments/assignment-repository';
+import { ClassRepository } from '../../../src/data/classes/class-repository';
+
+describe('SubmissionRepository', () => {
+    let submissionRepository: SubmissionRepository;
+    let studentRepository: StudentRepository;
+    let groupRepository: GroupRepository;
+    let assignmentRepository: AssignmentRepository;
+    let classRepository: ClassRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        submissionRepository = getSubmissionRepository();
+        studentRepository = getStudentRepository();
+        groupRepository = getGroupRepository();
+        assignmentRepository = getAssignmentRepository();
+        classRepository = getClassRepository();
+    });
+
+    it('should find the requested submission', async () => {
+        const id = new LearningObjectIdentifier('id03', Language.English, '1');
+        const submission = await submissionRepository.findSubmissionByLearningObjectAndSubmissionNumber(id, 1);
+
+        expect(submission).toBeTruthy();
+        expect(submission?.content).toBe('sub1');
+    });
+
+    it('should find the most recent submission for a student', async () => {
+        const id = new LearningObjectIdentifier('id02', Language.English, '1');
+        const student = await studentRepository.findByUsername('Noordkaap');
+        const submission = await submissionRepository.findMostRecentSubmissionForStudent(id, student!);
+
+        expect(submission).toBeTruthy();
+        expect(submission?.submissionTime.getDate()).toBe(25);
+    });
+
+    it('should find the most recent submission for a group', async () => {
+        const id = new LearningObjectIdentifier('id03', Language.English, '1');
+        const class_ = await classRepository.findById('id01');
+        const assignment = await assignmentRepository.findByClassAndId(class_!, 1);
+        const group = await groupRepository.findByAssignmentAndGroupNumber(assignment!, 1);
+        const submission = await submissionRepository.findMostRecentSubmissionForGroup(id, group!);
+
+        expect(submission).toBeTruthy();
+        expect(submission?.submissionTime.getDate()).toBe(25);
+    });
+
+    it('should not find a deleted submission', async () => {
+        const id = new LearningObjectIdentifier('id01', Language.English, '1');
+        await submissionRepository.deleteSubmissionByLearningObjectAndSubmissionNumber(id, 1);
+
+        const submission = await submissionRepository.findSubmissionByLearningObjectAndSubmissionNumber(id, 1);
+
+        expect(submission).toBeNull();
+    });
+});
--- a/backend/tests/data/classes/class-join-request.test.ts
+++ b/backend/tests/data/classes/class-join-request.test.ts
@ -0,0 +1,47 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests';
+import { ClassJoinRequestRepository } from '../../../src/data/classes/class-join-request-repository';
+import { getClassJoinRequestRepository, getClassRepository, getStudentRepository } from '../../../src/data/repositories';
+import { StudentRepository } from '../../../src/data/users/student-repository';
+import { Class } from '../../../src/entities/classes/class.entity';
+import { ClassRepository } from '../../../src/data/classes/class-repository';
+import { Student } from '../../../src/entities/users/student.entity';
+
+describe('ClassJoinRequestRepository', () => {
+    let classJoinRequestRepository: ClassJoinRequestRepository;
+    let studentRepository: StudentRepository;
+    let cassRepository: ClassRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        classJoinRequestRepository = getClassJoinRequestRepository();
+        studentRepository = getStudentRepository();
+        cassRepository = getClassRepository();
+    });
+
+    it('should list all requests from student to join classes', async () => {
+        const student = await studentRepository.findByUsername('PinkFloyd');
+        const requests = await classJoinRequestRepository.findAllRequestsBy(student!);
+
+        expect(requests).toBeTruthy();
+        expect(requests).toHaveLength(2);
+    });
+
+    it('should list all requests to a single class', async () => {
+        const class_ = await cassRepository.findById('id02');
+        const requests = await classJoinRequestRepository.findAllOpenRequestsTo(class_!);
+
+        expect(requests).toBeTruthy();
+        expect(requests).toHaveLength(2);
+    });
+
+    it('should not find a removed request', async () => {
+        const student = await studentRepository.findByUsername('SmashingPumpkins');
+        const class_ = await cassRepository.findById('id03');
+        await classJoinRequestRepository.deleteBy(student!, class_!);
+
+        const request = await classJoinRequestRepository.findAllRequestsBy(student!);
+
+        expect(request).toHaveLength(0);
+    });
+});
--- a/backend/tests/data/classes/classes.test.ts
+++ b/backend/tests/data/classes/classes.test.ts
@ -0,0 +1,34 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { ClassRepository } from '../../../src/data/classes/class-repository';
+import { setupTestApp } from '../../setup-tests';
+import { getClassRepository } from '../../../src/data/repositories';
+
+describe('ClassRepository', () => {
+    let classRepository: ClassRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        classRepository = getClassRepository();
+    });
+
+    it('should return nothing because id does not exist', async () => {
+        const classVar = await classRepository.findById('test_id');
+
+        expect(classVar).toBeNull();
+    });
+
+    it('should return requested class', async () => {
+        const classVar = await classRepository.findById('id01');
+
+        expect(classVar).toBeTruthy();
+        expect(classVar?.displayName).toBe('class01');
+    });
+
+    it('class should be gone after deletion', async () => {
+        await classRepository.deleteById('id04');
+
+        const classVar = await classRepository.findById('id04');
+
+        expect(classVar).toBeNull();
+    });
+});
--- a/backend/tests/data/classes/teacher-invitation.test.ts
+++ b/backend/tests/data/classes/teacher-invitation.test.ts
@ -0,0 +1,54 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests';
+import { getClassRepository, getTeacherInvitationRepository, getTeacherRepository } from '../../../src/data/repositories';
+import { TeacherInvitationRepository } from '../../../src/data/classes/teacher-invitation-repository';
+import { TeacherRepository } from '../../../src/data/users/teacher-repository';
+import { ClassRepository } from '../../../src/data/classes/class-repository';
+
+describe('ClassRepository', () => {
+    let teacherInvitationRepository: TeacherInvitationRepository;
+    let teacherRepository: TeacherRepository;
+    let classRepository: ClassRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        teacherInvitationRepository = getTeacherInvitationRepository();
+        teacherRepository = getTeacherRepository();
+        classRepository = getClassRepository();
+    });
+
+    it('should return all invitations from a teacher', async () => {
+        const teacher = await teacherRepository.findByUsername('LimpBizkit');
+        const invitations = await teacherInvitationRepository.findAllInvitationsBy(teacher!);
+
+        expect(invitations).toBeTruthy();
+        expect(invitations).toHaveLength(2);
+    });
+
+    it('should return all invitations for a teacher', async () => {
+        const teacher = await teacherRepository.findByUsername('FooFighters');
+        const invitations = await teacherInvitationRepository.findAllInvitationsFor(teacher!);
+
+        expect(invitations).toBeTruthy();
+        expect(invitations).toHaveLength(2);
+    });
+
+    it('should return all invitations for a class', async () => {
+        const class_ = await classRepository.findById('id02');
+        const invitations = await teacherInvitationRepository.findAllInvitationsForClass(class_!);
+
+        expect(invitations).toBeTruthy();
+        expect(invitations).toHaveLength(2);
+    });
+
+    it('should not find a removed invitation', async () => {
+        const class_ = await classRepository.findById('id01');
+        const sender = await teacherRepository.findByUsername('FooFighters');
+        const receiver = await teacherRepository.findByUsername('LimpBizkit');
+        await teacherInvitationRepository.deleteBy(class_!, sender!, receiver!);
+
+        const invitation = await teacherInvitationRepository.findAllInvitationsBy(sender!);
+
+        expect(invitation).toHaveLength(0);
+    });
+});
--- a/backend/tests/data/content/attachment-repository.test.ts
+++ b/backend/tests/data/content/attachment-repository.test.ts
@ -0,0 +1,79 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests.js';
+import { getAttachmentRepository, getLearningObjectRepository } from '../../../src/data/repositories.js';
+import { AttachmentRepository } from '../../../src/data/content/attachment-repository.js';
+import { LearningObjectRepository } from '../../../src/data/content/learning-object-repository.js';
+import example from '../../test-assets/learning-objects/pn-werkingnotebooks/pn-werkingnotebooks-example.js';
+import { LearningObject } from '../../../src/entities/content/learning-object.entity.js';
+import { Attachment } from '../../../src/entities/content/attachment.entity.js';
+import { LearningObjectIdentifier } from '../../../src/entities/content/learning-object-identifier.js';
+
+const NEWER_TEST_SUFFIX = 'nEweR';
+
+function createTestLearningObjects(learningObjectRepo: LearningObjectRepository): { older: LearningObject; newer: LearningObject } {
+    const olderExample = example.createLearningObject();
+    learningObjectRepo.save(olderExample);
+
+    const newerExample = example.createLearningObject();
+    newerExample.title = 'Newer example';
+    newerExample.version = 100;
+
+    return {
+        older: olderExample,
+        newer: newerExample,
+    };
+}
+
+describe('AttachmentRepository', () => {
+    let attachmentRepo: AttachmentRepository;
+    let exampleLearningObjects: { older: LearningObject; newer: LearningObject };
+    let attachmentsOlderLearningObject: Attachment[];
+
+    beforeAll(async () => {
+        await setupTestApp();
+        attachmentRepo = getAttachmentRepository();
+        exampleLearningObjects = createTestLearningObjects(getLearningObjectRepository());
+    });
+
+    it('can add attachments to learning objects without throwing an error', () => {
+        attachmentsOlderLearningObject = Object.values(example.createAttachment).map((fn) => fn(exampleLearningObjects.older));
+
+        for (const attachment of attachmentsOlderLearningObject) {
+            attachmentRepo.save(attachment);
+        }
+    });
+
+    let attachmentOnlyNewer: Attachment;
+    it('allows us to add attachments with the same name to a different learning object without throwing an error', () => {
+        attachmentOnlyNewer = Object.values(example.createAttachment)[0](exampleLearningObjects.newer);
+        attachmentOnlyNewer.content.write(NEWER_TEST_SUFFIX);
+
+        attachmentRepo.save(attachmentOnlyNewer);
+    });
+
+    let olderLearningObjectId: LearningObjectIdentifier;
+    it('returns the correct attachment when queried by learningObjectId and attachment name', async () => {
+        olderLearningObjectId = {
+            hruid: exampleLearningObjects.older.hruid,
+            language: exampleLearningObjects.older.language,
+            version: exampleLearningObjects.older.version,
+        };
+
+        const result = await attachmentRepo.findByLearningObjectIdAndName(olderLearningObjectId, attachmentsOlderLearningObject[0].name);
+        expect(result).toBe(attachmentsOlderLearningObject[0]);
+    });
+
+    it('returns null when queried by learningObjectId and non-existing attachment name', async () => {
+        const result = await attachmentRepo.findByLearningObjectIdAndName(olderLearningObjectId, 'non-existing name');
+        expect(result).toBe(null);
+    });
+
+    it('returns the newer version of the attachment when only queried by hruid, language and attachment name (but not version)', async () => {
+        const result = await attachmentRepo.findByMostRecentVersionOfLearningObjectAndName(
+            exampleLearningObjects.older.hruid,
+            exampleLearningObjects.older.language,
+            attachmentOnlyNewer.name
+        );
+        expect(result).toBe(attachmentOnlyNewer);
+    });
+});
--- a/backend/tests/data/content/attachments.test.ts
+++ b/backend/tests/data/content/attachments.test.ts
@ -0,0 +1,31 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests.js';
+import { getAttachmentRepository, getLearningObjectRepository } from '../../../src/data/repositories.js';
+import { AttachmentRepository } from '../../../src/data/content/attachment-repository.js';
+import { LearningObjectRepository } from '../../../src/data/content/learning-object-repository.js';
+import { LearningObjectIdentifier } from '../../../src/entities/content/learning-object-identifier.js';
+import { Language } from '../../../src/entities/content/language.js';
+
+describe('AttachmentRepository', () => {
+    let attachmentRepository: AttachmentRepository;
+    let learningObjectRepository: LearningObjectRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        attachmentRepository = getAttachmentRepository();
+        learningObjectRepository = getLearningObjectRepository();
+    });
+
+    it('should return the requested attachment', async () => {
+        const id = new LearningObjectIdentifier('id02', Language.English, '1');
+        const learningObject = await learningObjectRepository.findByIdentifier(id);
+
+        const attachment = await attachmentRepository.findByMostRecentVersionOfLearningObjectAndName(
+            learningObject!,
+            Language.English,
+            'attachment01'
+        );
+
+        expect(attachment).toBeTruthy();
+    });
+});
--- a/backend/tests/data/content/learning-object-repository.test.ts
+++ b/backend/tests/data/content/learning-object-repository.test.ts
@ -0,0 +1,72 @@
+import { beforeAll, describe, it, expect } from 'vitest';
+import { LearningObjectRepository } from '../../../src/data/content/learning-object-repository.js';
+import { setupTestApp } from '../../setup-tests.js';
+import { getLearningObjectRepository } from '../../../src/data/repositories.js';
+import example from '../../test-assets/learning-objects/pn-werkingnotebooks/pn-werkingnotebooks-example.js';
+import { LearningObject } from '../../../src/entities/content/learning-object.entity.js';
+import { expectToBeCorrectEntity } from '../../test-utils/expectations.js';
+
+describe('LearningObjectRepository', () => {
+    let learningObjectRepository: LearningObjectRepository;
+
+    let exampleLearningObject: LearningObject;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        learningObjectRepository = getLearningObjectRepository();
+    });
+
+    it('should be able to add a learning object to it without an error', async () => {
+        exampleLearningObject = example.createLearningObject();
+        await learningObjectRepository.insert(exampleLearningObject);
+    });
+
+    it('should return the learning object when queried by id', async () => {
+        const result = await learningObjectRepository.findByIdentifier({
+            hruid: exampleLearningObject.hruid,
+            language: exampleLearningObject.language,
+            version: exampleLearningObject.version,
+        });
+        expect(result).toBeInstanceOf(LearningObject);
+        expectToBeCorrectEntity(
+            {
+                name: 'actual',
+                entity: result!,
+            },
+            {
+                name: 'expected',
+                entity: exampleLearningObject,
+            }
+        );
+    });
+
+    it('should return null when non-existing version is queried', async () => {
+        const result = await learningObjectRepository.findByIdentifier({
+            hruid: exampleLearningObject.hruid,
+            language: exampleLearningObject.language,
+            version: 100,
+        });
+        expect(result).toBe(null);
+    });
+
+    let newerExample: LearningObject;
+
+    it('should allow a learning object with the same id except a different version to be added', async () => {
+        newerExample = example.createLearningObject();
+        newerExample.version = 10;
+        newerExample.title += ' (nieuw)';
+        await learningObjectRepository.save(newerExample);
+    });
+
+    it('should return the newest version of the learning object when queried by only hruid and language', async () => {
+        const result = await learningObjectRepository.findLatestByHruidAndLanguage(newerExample.hruid, newerExample.language);
+        expect(result).toBeInstanceOf(LearningObject);
+        expect(result?.version).toBe(10);
+        expect(result?.title).toContain('(nieuw)');
+    });
+
+    it('should return null when queried by non-existing hruid or language', async () => {
+        const result = await learningObjectRepository.findLatestByHruidAndLanguage('something_that_does_not_exist', exampleLearningObject.language);
+        expect(result).toBe(null);
+    });
+});
--- a/backend/tests/data/content/learning-objects.test.ts
+++ b/backend/tests/data/content/learning-objects.test.ts
@ -0,0 +1,32 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { LearningObjectRepository } from '../../../src/data/content/learning-object-repository';
+import { getLearningObjectRepository } from '../../../src/data/repositories';
+import { setupTestApp } from '../../setup-tests';
+import { LearningObjectIdentifier } from '../../../src/entities/content/learning-object-identifier';
+import { Language } from '../../../src/entities/content/language';
+
+describe('LearningObjectRepository', () => {
+    let learningObjectRepository: LearningObjectRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        learningObjectRepository = getLearningObjectRepository();
+    });
+
+    const id01 = new LearningObjectIdentifier('id01', Language.English, '1');
+    const id02 = new LearningObjectIdentifier('test_id', Language.English, '1');
+
+    it('should return the learning object that matches identifier 1', async () => {
+        const learningObject = await learningObjectRepository.findByIdentifier(id01);
+
+        expect(learningObject).toBeTruthy();
+        expect(learningObject?.title).toBe('Undertow');
+        expect(learningObject?.description).toBe('debute');
+    });
+
+    it('should return nothing because the identifier does not exist in the database', async () => {
+        const learningObject = await learningObjectRepository.findByIdentifier(id02);
+
+        expect(learningObject).toBeNull();
+    });
+});
--- a/backend/tests/data/content/learning-path-repository.test.ts
+++ b/backend/tests/data/content/learning-path-repository.test.ts
@ -0,0 +1,66 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests.js';
+import { getLearningPathRepository } from '../../../src/data/repositories.js';
+import { LearningPathRepository } from '../../../src/data/content/learning-path-repository.js';
+import example from '../../test-assets/learning-paths/pn-werking-example.js';
+import { LearningPath } from '../../../src/entities/content/learning-path.entity.js';
+import { expectToBeCorrectEntity } from '../../test-utils/expectations.js';
+import { Language } from '../../../src/entities/content/language.js';
+
+function expectToHaveFoundPrecisely(expected: LearningPath, result: LearningPath[]): void {
+    expect(result).toHaveProperty('length');
+    expect(result.length).toBe(1);
+    expectToBeCorrectEntity({ entity: result[0]! }, { entity: expected });
+}
+
+function expectToHaveFoundNothing(result: LearningPath[]): void {
+    expect(result).toHaveProperty('length');
+    expect(result.length).toBe(0);
+}
+
+describe('LearningPathRepository', () => {
+    let learningPathRepo: LearningPathRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        learningPathRepo = getLearningPathRepository();
+    });
+
+    let examplePath: LearningPath;
+
+    it('should be able to add a learning path without throwing an error', async () => {
+        examplePath = example.createLearningPath();
+        await learningPathRepo.insert(examplePath);
+    });
+
+    it('should return the added path when it is queried by hruid and language', async () => {
+        const result = await learningPathRepo.findByHruidAndLanguage(examplePath.hruid, examplePath.language);
+        expect(result).toBeInstanceOf(LearningPath);
+        expectToBeCorrectEntity({ entity: result! }, { entity: examplePath });
+    });
+
+    it('should return null to a query on a non-existing hruid or language', async () => {
+        const result = await learningPathRepo.findByHruidAndLanguage('not_existing_hruid', examplePath.language);
+        expect(result).toBe(null);
+    });
+
+    it('should return the learning path when we search for a search term occurring in its title', async () => {
+        const result = await learningPathRepo.findByQueryStringAndLanguage(examplePath.title.slice(4, 9), examplePath.language);
+        expectToHaveFoundPrecisely(examplePath, result);
+    });
+
+    it('should return the learning path when we search for a search term occurring in its description', async () => {
+        const result = await learningPathRepo.findByQueryStringAndLanguage(examplePath.description.slice(8, 15), examplePath.language);
+        expectToHaveFoundPrecisely(examplePath, result);
+    });
+
+    it('should return null when we search for something not occurring in its title or description', async () => {
+        const result = await learningPathRepo.findByQueryStringAndLanguage('something not occurring in the path', examplePath.language);
+        expectToHaveFoundNothing(result);
+    });
+
+    it('should return null when we search for something occurring in its title, but another language', async () => {
+        const result = await learningPathRepo.findByQueryStringAndLanguage(examplePath.description.slice(1, 3), Language.Kalaallisut);
+        expectToHaveFoundNothing(result);
+    });
+});
--- a/backend/tests/data/content/learning-paths.test.ts
+++ b/backend/tests/data/content/learning-paths.test.ts
@ -0,0 +1,28 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { getLearningPathRepository } from '../../../src/data/repositories';
+import { LearningPathRepository } from '../../../src/data/content/learning-path-repository';
+import { setupTestApp } from '../../setup-tests';
+import { Language } from '../../../src/entities/content/language';
+
+describe('LearningPathRepository', () => {
+    let learningPathRepository: LearningPathRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        learningPathRepository = getLearningPathRepository();
+    });
+
+    it('should return nothing because no match for hruid and language', async () => {
+        const learningPath = await learningPathRepository.findByHruidAndLanguage('test_id', Language.Dutch);
+
+        expect(learningPath).toBeNull();
+    });
+
+    it('should return requested learning path', async () => {
+        const learningPath = await learningPathRepository.findByHruidAndLanguage('id01', Language.English);
+
+        expect(learningPath).toBeTruthy();
+        expect(learningPath?.title).toBe('repertoire Tool');
+        expect(learningPath?.description).toBe('all about Tool');
+    });
+});
--- a/backend/tests/data/questions/answers.test.ts
+++ b/backend/tests/data/questions/answers.test.ts
@ -0,0 +1,66 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests';
+import { AnswerRepository } from '../../../src/data/questions/answer-repository';
+import { getAnswerRepository, getQuestionRepository, getTeacherRepository } from '../../../src/data/repositories';
+import { QuestionRepository } from '../../../src/data/questions/question-repository';
+import { LearningObjectIdentifier } from '../../../src/entities/content/learning-object-identifier';
+import { Language } from '../../../src/entities/content/language';
+import { TeacherRepository } from '../../../src/data/users/teacher-repository';
+
+describe('AnswerRepository', () => {
+    let answerRepository: AnswerRepository;
+    let questionRepository: QuestionRepository;
+    let teacherRepository: TeacherRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        answerRepository = getAnswerRepository();
+        questionRepository = getQuestionRepository();
+        teacherRepository = getTeacherRepository();
+    });
+
+    it('should find all answers to a question', async () => {
+        const id = new LearningObjectIdentifier('id05', Language.English, '1');
+        const questions = await questionRepository.findAllQuestionsAboutLearningObject(id);
+
+        const question = questions.filter((it) => it.sequenceNumber == 2)[0];
+
+        const answers = await answerRepository.findAllAnswersToQuestion(question);
+
+        expect(answers).toBeTruthy();
+        expect(answers).toHaveLength(2);
+        expect(answers[0].content).toBeOneOf(['answer', 'answer2']);
+        expect(answers[1].content).toBeOneOf(['answer', 'answer2']);
+    });
+
+    it('should create an answer to a question', async () => {
+        const teacher = await teacherRepository.findByUsername('FooFighters');
+        const id = new LearningObjectIdentifier('id05', Language.English, '1');
+        const questions = await questionRepository.findAllQuestionsAboutLearningObject(id);
+
+        const question = questions[0];
+
+        await answerRepository.createAnswer({
+            toQuestion: question,
+            author: teacher!,
+            content: 'created answer',
+        });
+
+        const answers = await answerRepository.findAllAnswersToQuestion(question);
+
+        expect(answers).toBeTruthy();
+        expect(answers).toHaveLength(1);
+        expect(answers[0].content).toBe('created answer');
+    });
+
+    it('should not find a removed answer', async () => {
+        const id = new LearningObjectIdentifier('id04', Language.English, '1');
+        const questions = await questionRepository.findAllQuestionsAboutLearningObject(id);
+
+        await answerRepository.removeAnswerByQuestionAndSequenceNumber(questions[0], 1);
+
+        const emptyList = await answerRepository.findAllAnswersToQuestion(questions[0]);
+
+        expect(emptyList).toHaveLength(0);
+    });
+});
--- a/backend/tests/data/questions/questions.test.ts
+++ b/backend/tests/data/questions/questions.test.ts
@ -0,0 +1,52 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests';
+import { QuestionRepository } from '../../../src/data/questions/question-repository';
+import { getLearningObjectRepository, getQuestionRepository, getStudentRepository } from '../../../src/data/repositories';
+import { StudentRepository } from '../../../src/data/users/student-repository';
+import { LearningObjectRepository } from '../../../src/data/content/learning-object-repository';
+import { LearningObjectIdentifier } from '../../../src/entities/content/learning-object-identifier';
+import { Language } from '../../../src/entities/content/language';
+
+describe('QuestionRepository', () => {
+    let questionRepository: QuestionRepository;
+    let studentRepository: StudentRepository;
+    let learningObjectRepository: LearningObjectRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        questionRepository = getQuestionRepository();
+        studentRepository = getStudentRepository();
+        learningObjectRepository = getLearningObjectRepository();
+    });
+
+    it('should return all questions part of the given learning object', async () => {
+        const id = new LearningObjectIdentifier('id05', Language.English, '1');
+        const questions = await questionRepository.findAllQuestionsAboutLearningObject(id);
+
+        expect(questions).toBeTruthy();
+        expect(questions).toHaveLength(2);
+    });
+
+    it('should create new question', async () => {
+        const id = new LearningObjectIdentifier('id03', Language.English, '1');
+        const student = await studentRepository.findByUsername('Noordkaap');
+        await questionRepository.createQuestion({
+            loId: id,
+            author: student!,
+            content: 'question?',
+        });
+        const question = await questionRepository.findAllQuestionsAboutLearningObject(id);
+
+        expect(question).toBeTruthy();
+        expect(question).toHaveLength(1);
+    });
+
+    it('should not find removed question', async () => {
+        const id = new LearningObjectIdentifier('id04', Language.English, '1');
+        await questionRepository.removeQuestionByLearningObjectAndSequenceNumber(id, 1);
+
+        const question = await questionRepository.findAllQuestionsAboutLearningObject(id);
+
+        expect(question).toHaveLength(0);
+    });
+});
--- a/backend/tests/data/users/students.test.ts
+++ b/backend/tests/data/users/students.test.ts
@ -1,8 +1,8 @@
-import { setupTestApp } from '../setup-tests.js';
-import { Student } from '../../src/entities/users/student.entity.js';
+import { setupTestApp } from '../../setup-tests.js';
+import { Student } from '../../../src/entities/users/student.entity.js';
 import { describe, it, expect, beforeAll } from 'vitest';
-import { StudentRepository } from '../../src/data/users/student-repository.js';
-import { getStudentRepository } from '../../src/data/repositories.js';
+import { StudentRepository } from '../../../src/data/users/student-repository.js';
+import { getStudentRepository } from '../../../src/data/repositories.js';

 const username = 'teststudent';
 const firstName = 'John';
@ -15,6 +15,20 @@ describe('StudentRepository', () => {
        studentRepository = getStudentRepository();
    });

+    it('should not return a student because username does not exist', async () => {
+        const student = await studentRepository.findByUsername('test');
+
+        expect(student).toBeNull();
+    });
+
+    it('should return student from the datbase', async () => {
+        const student = await studentRepository.findByUsername('Noordkaap');
+
+        expect(student).toBeTruthy();
+        expect(student?.firstName).toBe('Stijn');
+        expect(student?.lastName).toBe('Meuris');
+    });
+
    it('should return the queried student after he was added', async () => {
        await studentRepository.insert(new Student(username, firstName, lastName));

--- a/backend/tests/data/users/teachers.test.ts
+++ b/backend/tests/data/users/teachers.test.ts
@ -0,0 +1,47 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { TeacherRepository } from '../../../src/data/users/teacher-repository';
+import { setupTestApp } from '../../setup-tests';
+import { getTeacherRepository } from '../../../src/data/repositories';
+import { Teacher } from '../../../src/entities/users/teacher.entity';
+
+const username = 'testteacher';
+const firstName = 'John';
+const lastName = 'Doe';
+describe('TeacherRepository', () => {
+    let teacherRepository: TeacherRepository;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        teacherRepository = getTeacherRepository();
+    });
+
+    it('should not return a teacher because username does not exist', async () => {
+        const teacher = await teacherRepository.findByUsername('test');
+
+        expect(teacher).toBeNull();
+    });
+
+    it('should return teacher from the datbase', async () => {
+        const teacher = await teacherRepository.findByUsername('FooFighters');
+
+        expect(teacher).toBeTruthy();
+        expect(teacher?.firstName).toBe('Dave');
+        expect(teacher?.lastName).toBe('Grohl');
+    });
+
+    it('should return the queried teacher after he was added', async () => {
+        await teacherRepository.insert(new Teacher(username, firstName, lastName));
+
+        const retrievedTeacher = await teacherRepository.findByUsername(username);
+        expect(retrievedTeacher).toBeTruthy();
+        expect(retrievedTeacher?.firstName).toBe(firstName);
+        expect(retrievedTeacher?.lastName).toBe(lastName);
+    });
+
+    it('should no longer return the queried teacher after he was removed again', async () => {
+        await teacherRepository.deleteByUsername('ZesdeMetaal');
+
+        const retrievedTeacher = await teacherRepository.findByUsername('ZesdeMetaal');
+        expect(retrievedTeacher).toBeNull();
+    });
+});
--- a/backend/tests/service/learning-objects.test.ts
+++ b/backend/tests/service/learning-objects.test.ts
@ -0,0 +1,117 @@
+import { describe, it, expect, vi } from 'vitest';
+import { LearningObjectMetadata, LearningPath } from '../../src/interfaces/learningPath';
+import { fetchWithLogging } from '../../src/util/apiHelper';
+import { getLearningObjectById, getLearningObjectsFromPath } from '../../src/services/learningObjects';
+import { fetchLearningPaths } from '../../src/services/learningPaths';
+
+// Mock API functions
+vi.mock('../../src/util/apiHelper', () => ({
+    fetchWithLogging: vi.fn(),
+}));
+
+vi.mock('../../src/services/learningPaths', () => ({
+    fetchLearningPaths: vi.fn(),
+}));
+
+describe('getLearningObjectById', () => {
+    const hruid = 'test-object';
+    const language = 'en';
+    const mockMetadata: LearningObjectMetadata = {
+        hruid,
+        _id: '123',
+        uuid: 'uuid-123',
+        version: 1,
+        title: 'Test Object',
+        language,
+        difficulty: 5,
+        estimated_time: 120,
+        available: true,
+        teacher_exclusive: false,
+        educational_goals: [{ source: 'source', id: 'id' }],
+        keywords: ['robotics'],
+        description: 'A test object',
+        target_ages: [10, 12],
+        content_type: 'markdown',
+        content_location: '',
+    };
+
+    it('✅ Should return a filtered learning object when API provides data', async () => {
+        vi.mocked(fetchWithLogging).mockResolvedValueOnce(mockMetadata);
+
+        const result = await getLearningObjectById(hruid, language);
+
+        expect(result).toEqual({
+            key: hruid,
+            _id: '123',
+            uuid: 'uuid-123',
+            version: 1,
+            title: 'Test Object',
+            htmlUrl: expect.stringContaining('/learningObject/getRaw?hruid=test-object&language=en'),
+            language,
+            difficulty: 5,
+            estimatedTime: 120,
+            available: true,
+            teacherExclusive: false,
+            educationalGoals: [{ source: 'source', id: 'id' }],
+            keywords: ['robotics'],
+            description: 'A test object',
+            targetAges: [10, 12],
+            contentType: 'markdown',
+            contentLocation: '',
+        });
+    });
+
+    it('⚠️ Should return null if API returns no metadata', async () => {
+        vi.mocked(fetchWithLogging).mockResolvedValueOnce(null);
+        const result = await getLearningObjectById(hruid, language);
+        expect(result).toBeNull();
+    });
+});
+
+describe('getLearningObjectsFromPath', () => {
+    const hruid = 'test-path';
+    const language = 'en';
+
+    it('✅ Should not give error or warning', async () => {
+        const mockPathResponse: LearningPath[] = [
+            {
+                _id: 'path-1',
+                hruid,
+                language,
+                title: 'Test Path',
+                description: '',
+                num_nodes: 1,
+                num_nodes_left: 0,
+                nodes: [],
+                keywords: '',
+                target_ages: [],
+                min_age: 10,
+                max_age: 12,
+                __order: 1,
+            },
+        ];
+
+        vi.mocked(fetchLearningPaths).mockResolvedValueOnce({
+            success: true,
+            source: 'Test Source',
+            data: mockPathResponse,
+        });
+
+        const result = await getLearningObjectsFromPath(hruid, language);
+        expect(result).toEqual([]);
+    });
+
+    it('⚠️ Should give a warning', async () => {
+        vi.mocked(fetchLearningPaths).mockResolvedValueOnce({ success: false, source: 'Test Source', data: [] });
+
+        const result = await getLearningObjectsFromPath(hruid, language);
+        expect(result).toEqual([]);
+    });
+
+    it('❌ Should give an error', async () => {
+        vi.mocked(fetchLearningPaths).mockRejectedValueOnce(new Error('API Error'));
+
+        const result = await getLearningObjectsFromPath(hruid, language);
+        expect(result).toEqual([]);
+    });
+});
--- a/backend/tests/service/learning-paths.test.ts
+++ b/backend/tests/service/learning-paths.test.ts
@ -0,0 +1,90 @@
+import { describe, it, expect, vi } from 'vitest';
+import { fetchLearningPaths, searchLearningPaths } from '../../src/services/learningPaths';
+import { fetchWithLogging } from '../../src/util/apiHelper';
+import { LearningPathResponse } from '../../src/interfaces/learningPath';
+
+// Mock the fetchWithLogging module using vi
+vi.mock('../../src/util/apiHelper', () => ({
+    fetchWithLogging: vi.fn(),
+}));
+
+describe('fetchLearningPaths', () => {
+    // Mock data and response
+    const mockHruids = ['pn_werking', 'art1'];
+    const language = 'en';
+    const source = 'Test Source';
+    const mockResponse = [{ title: 'Test Path', hruids: mockHruids }];
+
+    it('✅ Should return a successful response when HRUIDs are provided', async () => {
+        // Mock the function to return mockResponse
+        vi.mocked(fetchWithLogging).mockResolvedValue(mockResponse);
+
+        const result: LearningPathResponse = await fetchLearningPaths(mockHruids, language, source);
+
+        expect(result.success).toBe(true);
+        expect(result.data).toEqual(mockResponse);
+        expect(result.source).toBe(source);
+    });
+
+    it('⚠️ Should return an error when no HRUIDs are provided', async () => {
+        vi.mocked(fetchWithLogging).mockResolvedValue(mockResponse);
+
+        const result: LearningPathResponse = await fetchLearningPaths([], language, source);
+
+        expect(result.success).toBe(false);
+        expect(result.data).toBeNull();
+        expect(result.message).toBe(`No HRUIDs provided for ${source}.`);
+    });
+
+    it('⚠️ Should return a failure response when no learning paths are found', async () => {
+        // Mock fetchWithLogging to return an empty array
+        vi.mocked(fetchWithLogging).mockResolvedValue([]);
+
+        const result: LearningPathResponse = await fetchLearningPaths(mockHruids, language, source);
+
+        expect(result.success).toBe(false);
+        expect(result.data).toEqual([]);
+        expect(result.message).toBe(`No learning paths found for ${source}.`);
+    });
+});
+
+describe('searchLearningPaths', () => {
+    const query =
+        'https://dwengo.org/backend/api/learningPath/getPathsFromIdList?pathIdList=%7B%22hruids%22:%5B%22pn_werking%22,%22un_artificiele_intelligentie%22%5D%7D&language=nl';
+    const language = 'nl';
+
+    it('✅ Should return search results when API responds with data', async () => {
+        const mockResults = [
+            {
+                _id: '67b4488c9dadb305c4104618',
+                language: 'nl',
+                hruid: 'pn_werking',
+                title: 'Werken met notebooks',
+                description: 'Een korte inleiding tot Python notebooks. Hoe ga je gemakkelijk en efficiënt met de notebooks aan de slag?',
+                num_nodes: 0,
+                num_nodes_left: 0,
+                nodes: [],
+                keywords: 'Python KIKS Wiskunde STEM AI',
+                target_ages: [14, 15, 16, 17, 18],
+                min_age: 14,
+                max_age: 18,
+                __order: 0,
+            },
+        ];
+
+        // Mock fetchWithLogging to return search results
+        vi.mocked(fetchWithLogging).mockResolvedValue(mockResults);
+
+        const result = await searchLearningPaths(query, language);
+
+        expect(result).toEqual(mockResults);
+    });
+
+    it('⚠️ Should return an empty array when API returns no results', async () => {
+        vi.mocked(fetchWithLogging).mockResolvedValue([]);
+
+        const result = await searchLearningPaths(query, language);
+
+        expect(result).toEqual([]);
+    });
+});
--- a/backend/tests/services/learning-objects/database-learning-object-provider.test.ts
+++ b/backend/tests/services/learning-objects/database-learning-object-provider.test.ts
@ -0,0 +1,108 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests';
+import { getLearningObjectRepository, getLearningPathRepository } from '../../../src/data/repositories';
+import example from '../../test-assets/learning-objects/pn-werkingnotebooks/pn-werkingnotebooks-example';
+import { LearningObject } from '../../../src/entities/content/learning-object.entity';
+import databaseLearningObjectProvider from '../../../src/services/learning-objects/database-learning-object-provider';
+import { expectToBeCorrectFilteredLearningObject } from '../../test-utils/expectations';
+import { FilteredLearningObject } from '../../../src/interfaces/learning-content';
+import { Language } from '../../../src/entities/content/language';
+import learningObjectExample from '../../test-assets/learning-objects/pn-werkingnotebooks/pn-werkingnotebooks-example';
+import learningPathExample from '../../test-assets/learning-paths/pn-werking-example';
+import { LearningPath } from '../../../src/entities/content/learning-path.entity';
+
+async function initExampleData(): Promise<{ learningObject: LearningObject; learningPath: LearningPath }> {
+    const learningObjectRepo = getLearningObjectRepository();
+    const learningPathRepo = getLearningPathRepository();
+    const learningObject = learningObjectExample.createLearningObject();
+    const learningPath = learningPathExample.createLearningPath();
+    await learningObjectRepo.save(learningObject);
+    await learningPathRepo.save(learningPath);
+    return { learningObject, learningPath };
+}
+
+const EXPECTED_TITLE_FROM_DWENGO_LEARNING_OBJECT = 'Notebook opslaan';
+
+describe('DatabaseLearningObjectProvider', () => {
+    let exampleLearningObject: LearningObject;
+    let exampleLearningPath: LearningPath;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        const exampleData = await initExampleData();
+        exampleLearningObject = exampleData.learningObject;
+        exampleLearningPath = exampleData.learningPath;
+    });
+    describe('getLearningObjectById', () => {
+        it('should return the learning object when it is queried by its id', async () => {
+            const result: FilteredLearningObject | null = await databaseLearningObjectProvider.getLearningObjectById(exampleLearningObject);
+            expect(result).toBeTruthy();
+            expectToBeCorrectFilteredLearningObject(result!, exampleLearningObject);
+        });
+
+        it('should return the learning object when it is queried by only hruid and language (but not version)', async () => {
+            const result: FilteredLearningObject | null = await databaseLearningObjectProvider.getLearningObjectById({
+                hruid: exampleLearningObject.hruid,
+                language: exampleLearningObject.language,
+            });
+            expect(result).toBeTruthy();
+            expectToBeCorrectFilteredLearningObject(result!, exampleLearningObject);
+        });
+
+        it('should return null when queried with an id that does not exist', async () => {
+            const result: FilteredLearningObject | null = await databaseLearningObjectProvider.getLearningObjectById({
+                hruid: 'non_existing_hruid',
+                language: Language.Dutch,
+            });
+            expect(result).toBeNull();
+        });
+    });
+    describe('getLearningObjectHTML', () => {
+        it('should return the correct rendering of the learning object', async () => {
+            const result = await databaseLearningObjectProvider.getLearningObjectHTML(exampleLearningObject);
+            expect(result).toEqual(example.getHTMLRendering());
+        });
+        it('should return null for a non-existing learning object', async () => {
+            const result = await databaseLearningObjectProvider.getLearningObjectHTML({
+                hruid: 'non_existing_hruid',
+                language: Language.Dutch,
+            });
+            expect(result).toBeNull();
+        });
+    });
+    describe('getLearningObjectIdsFromPath', () => {
+        it('should return all learning object IDs from a path', async () => {
+            const result = await databaseLearningObjectProvider.getLearningObjectIdsFromPath(exampleLearningPath);
+            expect(new Set(result)).toEqual(new Set(exampleLearningPath.nodes.map((it) => it.learningObjectHruid)));
+        });
+        it('should throw an error if queried with a path identifier for which there is no learning path', async () => {
+            await expect(
+                (async () => {
+                    await databaseLearningObjectProvider.getLearningObjectIdsFromPath({
+                        hruid: 'non_existing_hruid',
+                        language: Language.Dutch,
+                    });
+                })()
+            ).rejects.toThrowError();
+        });
+    });
+    describe('getLearningObjectsFromPath', () => {
+        it('should correctly return all learning objects which are on the path, even those who are not in the database', async () => {
+            const result = await databaseLearningObjectProvider.getLearningObjectsFromPath(exampleLearningPath);
+            expect(result.length).toBe(exampleLearningPath.nodes.length);
+            expect(new Set(result.map((it) => it.key))).toEqual(new Set(exampleLearningPath.nodes.map((it) => it.learningObjectHruid)));
+
+            expect(result.map((it) => it.title)).toContainEqual(EXPECTED_TITLE_FROM_DWENGO_LEARNING_OBJECT);
+        });
+        it('should throw an error if queried with a path identifier for which there is no learning path', async () => {
+            await expect(
+                (async () => {
+                    await databaseLearningObjectProvider.getLearningObjectsFromPath({
+                        hruid: 'non_existing_hruid',
+                        language: Language.Dutch,
+                    });
+                })()
+            ).rejects.toThrowError();
+        });
+    });
+});
--- a/backend/tests/services/learning-objects/learning-object-service.test.ts
+++ b/backend/tests/services/learning-objects/learning-object-service.test.ts
@ -0,0 +1,126 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { setupTestApp } from '../../setup-tests';
+import { LearningObject } from '../../../src/entities/content/learning-object.entity';
+import { getLearningObjectRepository, getLearningPathRepository } from '../../../src/data/repositories';
+import learningObjectExample from '../../test-assets/learning-objects/pn-werkingnotebooks/pn-werkingnotebooks-example';
+import learningObjectService from '../../../src/services/learning-objects/learning-object-service';
+import { LearningObjectIdentifier, LearningPathIdentifier } from '../../../src/interfaces/learning-content';
+import { Language } from '../../../src/entities/content/language';
+import { EnvVars, getEnvVar } from '../../../src/util/envvars';
+import { LearningPath } from '../../../src/entities/content/learning-path.entity';
+import learningPathExample from '../../test-assets/learning-paths/pn-werking-example';
+
+const EXPECTED_DWENGO_LEARNING_OBJECT_TITLE = 'Werken met notebooks';
+const DWENGO_TEST_LEARNING_OBJECT_ID: LearningObjectIdentifier = {
+    hruid: 'pn_werkingnotebooks',
+    language: Language.Dutch,
+    version: 3,
+};
+
+const DWENGO_TEST_LEARNING_PATH_ID: LearningPathIdentifier = {
+    hruid: 'pn_werking',
+    language: Language.Dutch,
+};
+const DWENGO_TEST_LEARNING_PATH_HRUIDS = new Set(['pn_werkingnotebooks', 'pn_werkingnotebooks2', 'pn_werkingnotebooks3']);
+
+async function initExampleData(): Promise<{ learningObject: LearningObject; learningPath: LearningPath }> {
+    const learningObjectRepo = getLearningObjectRepository();
+    const learningPathRepo = getLearningPathRepository();
+    const learningObject = learningObjectExample.createLearningObject();
+    const learningPath = learningPathExample.createLearningPath();
+    await learningObjectRepo.save(learningObject);
+    await learningPathRepo.save(learningPath);
+    return { learningObject, learningPath };
+}
+
+describe('LearningObjectService', () => {
+    let exampleLearningObject: LearningObject;
+    let exampleLearningPath: LearningPath;
+
+    beforeAll(async () => {
+        await setupTestApp();
+        const exampleData = await initExampleData();
+        exampleLearningObject = exampleData.learningObject;
+        exampleLearningPath = exampleData.learningPath;
+    });
+
+    describe('getLearningObjectById', () => {
+        it('returns the learning object from the Dwengo API if it does not have the user content prefix', async () => {
+            const result = await learningObjectService.getLearningObjectById(DWENGO_TEST_LEARNING_OBJECT_ID);
+            expect(result).not.toBeNull();
+            expect(result?.title).toBe(EXPECTED_DWENGO_LEARNING_OBJECT_TITLE);
+        });
+        it('returns the learning object from the database if it does have the user content prefix', async () => {
+            const result = await learningObjectService.getLearningObjectById(exampleLearningObject);
+            expect(result).not.toBeNull();
+            expect(result?.title).toBe(exampleLearningObject.title);
+        });
+        it('returns null if the hruid does not have the user content prefix and does not exist in the Dwengo repo', async () => {
+            const result = await learningObjectService.getLearningObjectById({
+                hruid: 'non-existing',
+                language: Language.Dutch,
+            });
+            expect(result).toBeNull();
+        });
+    });
+
+    describe('getLearningObjectHTML', () => {
+        it('returns the expected HTML when queried with the identifier of a learning object saved in the database', async () => {
+            const result = await learningObjectService.getLearningObjectHTML(exampleLearningObject);
+            expect(result).not.toBeNull();
+            expect(result).toEqual(learningObjectExample.getHTMLRendering());
+        });
+        it(
+            'returns the same HTML as the Dwengo API when queried with the identifier of a learning object that does ' +
+                'not start with the user content prefix',
+            async () => {
+                const result = await learningObjectService.getLearningObjectHTML(DWENGO_TEST_LEARNING_OBJECT_ID);
+                expect(result).not.toBeNull();
+
+                const responseFromDwengoApi = await fetch(
+                    getEnvVar(EnvVars.LearningContentRepoApiBaseUrl) +
+                        `/learningObject/getRaw?hruid=${DWENGO_TEST_LEARNING_OBJECT_ID.hruid}&language=${DWENGO_TEST_LEARNING_OBJECT_ID.language}&version=${DWENGO_TEST_LEARNING_OBJECT_ID.version}`
+                );
+                const responseHtml = await responseFromDwengoApi.text();
+                expect(result).toEqual(responseHtml);
+            }
+        );
+        it('returns null when queried with a non-existing identifier', async () => {
+            const result = await learningObjectService.getLearningObjectHTML({
+                hruid: 'non_existing_hruid',
+                language: Language.Dutch,
+            });
+            expect(result).toBeNull();
+        });
+    });
+
+    describe('getLearningObjectsFromPath', () => {
+        it('returns all learning objects when a learning path in the database is queried', async () => {
+            const result = await learningObjectService.getLearningObjectsFromPath(exampleLearningPath);
+            expect(result.map((it) => it.key)).toEqual(exampleLearningPath.nodes.map((it) => it.learningObjectHruid));
+        });
+        it('also returns all learning objects when a learning path from the Dwengo API is queried', async () => {
+            const result = await learningObjectService.getLearningObjectsFromPath(DWENGO_TEST_LEARNING_PATH_ID);
+            expect(new Set(result.map((it) => it.key))).toEqual(DWENGO_TEST_LEARNING_PATH_HRUIDS);
+        });
+        it('returns an empty list when queried with a non-existing learning path id', async () => {
+            const result = await learningObjectService.getLearningObjectsFromPath({ hruid: 'non_existing', language: Language.Dutch });
+            expect(result).toEqual([]);
+        });
+    });
+
+    describe('getLearningObjectIdsFromPath', () => {
+        it('returns all learning objects when a learning path in the database is queried', async () => {
+            const result = await learningObjectService.getLearningObjectIdsFromPath(exampleLearningPath);
+            expect(result).toEqual(exampleLearningPath.nodes.map((it) => it.learningObjectHruid));
+        });
+        it('also returns all learning object hruids when a learning path from the Dwengo API is queried', async () => {
+            const result = await learningObjectService.getLearningObjectIdsFromPath(DWENGO_TEST_LEARNING_PATH_ID);
+            expect(new Set(result)).toEqual(DWENGO_TEST_LEARNING_PATH_HRUIDS);
+        });
+        it('returns an empty list when queried with a non-existing learning path id', async () => {
+            const result = await learningObjectService.getLearningObjectIdsFromPath({ hruid: 'non_existing', language: Language.Dutch });
+            expect(result).toEqual([]);
+        });
+    });
+});
--- a/backend/tests/services/learning-objects/processing/processing-service.test.ts
+++ b/backend/tests/services/learning-objects/processing/processing-service.test.ts
@ -0,0 +1,25 @@
+import { describe, expect, it } from 'vitest';
+import mdExample from '../../../test-assets/learning-objects/pn-werkingnotebooks/pn-werkingnotebooks-example';
+import multipleChoiceExample from '../../../test-assets/learning-objects/test-multiple-choice/test-multiple-choice-example';
+import essayExample from '../../../test-assets/learning-objects/test-essay/test-essay-example';
+import processingService from '../../../../src/services/learning-objects/processing/processing-service';
+
+describe('ProcessingService', () => {
+    it('renders a markdown learning object correctly', async () => {
+        const markdownLearningObject = mdExample.createLearningObject();
+        const result = await processingService.render(markdownLearningObject);
+        expect(result).toEqual(mdExample.getHTMLRendering());
+    });
+
+    it('renders a multiple choice question correctly', async () => {
+        const multipleChoiceLearningObject = multipleChoiceExample.createLearningObject();
+        const result = await processingService.render(multipleChoiceLearningObject);
+        expect(result).toEqual(multipleChoiceExample.getHTMLRendering());
+    });
+
+    it('renders an essay question correctly', async () => {
+        const essayLearningObject = essayExample.createLearningObject();
+        const result = await processingService.render(essayLearningObject);
+        expect(result).toEqual(essayExample.getHTMLRendering());
+    });
+});
--- a/Show more
+++ b/Show more