fix(frontend): Frontend kan nu omgaan met foutieve leerpaden

- Geen of meerdere startnodes
- Transities naar een leerobject dat niet voorkomt
- Transities naar een leerobject dat meer dan één keer voorkomt.

frontend/src/data-objects/learning-paths/learning-path-node.ts

@@ -34,24 +34,21 @@ export class LearningPathNode {
             version: dto.version,
             language: dto.language,
             transitions: dto.transitions.map((transDto) => {
-                const nextNodeDto = otherNodes.filter(
+                const nextNodeDto = otherNodes.find(
                     (it) =>
                         it.learningobject_hruid === transDto.next.hruid &&
                         it.language === transDto.next.language &&
                         it.version === transDto.next.version,
                 );
-                if (nextNodeDto.length !== 1) {
+                if (nextNodeDto) {
-                    throw new Error(
+                    return {
-                        `Invalid learning path! There is a transition to node` +
+                        next: LearningPathNode.fromDTOAndOtherNodes(nextNodeDto, otherNodes),
-                            `${transDto.next.hruid}/${transDto.next.language}/${transDto.next.version}, but there are` +
+                        default: transDto.default,
-                            `${nextNodeDto.length} such nodes.`,
+                    };
-                    );
+                } else {
+                    return undefined
                 }
-                return {
+            }).filter(it => it !== undefined),
-                    next: LearningPathNode.fromDTOAndOtherNodes(nextNodeDto[0], otherNodes),
-                    default: transDto.default,
-                };
-            }),
             createdAt: new Date(dto.created_at),
             updatedAt: new Date(dto.updatedAt),
             done: dto.done,

frontend/src/data-objects/learning-paths/learning-path.ts

@@ -72,11 +72,6 @@ export class LearningPath {
     }
 
     static fromDTO(dto: LearningPathDTO): LearningPath {
-        const startNodeDto = dto.nodes.filter((it) => it.start_node === true);
-        if (startNodeDto.length !== 1) {
-            throw new Error(`Invalid learning path: ${dto.hruid}/${dto.language}!
-                                Expected precisely one start node, but there were ${startNodeDto.length}.`);
-        }
         return new LearningPath({
             language: dto.language,
             hruid: dto.hruid,
@@ -86,8 +81,17 @@ export class LearningPath {
             amountOfNodesLeft: dto.num_nodes_left,
             keywords: dto.keywords.split(" "),
             targetAges: { min: dto.min_age, max: dto.max_age },
-            startNode: LearningPathNode.fromDTOAndOtherNodes(startNodeDto[0], dto.nodes),
+            startNode: LearningPathNode.fromDTOAndOtherNodes(LearningPath.getStartNode(dto), dto.nodes),
             image: dto.image,
         });
     }
+
+    static getStartNode(dto: LearningPathDTO): LearningPathNodeDTO {
+        const startNodeDtos = dto.nodes.filter((it) => it.start_node === true);
+        if (startNodeDtos.length < 1) { // The learning path has no starting node -> use the first node.
+            return dto.nodes[0];
+        } else { // The learning path has 1 or more starting nodes -> use the first start node.
+            return startNodeDtos[0];
+        }
+    }
 }