How do you implement a DataLoader in NestJS and scope it correctly per request?

Q: How do you implement a DataLoader in NestJS and scope it correctly per request?

// npm i dataloader import DataLoader from 'dataloader'; @Injectable({ scope: Scope.REQUEST }) export class PostsDataLoader { constructor(private postsService: PostsService) {} // Batch function: receives all queued userIds from one tick readonly batchLoadPosts = new DataLoader ( async (userIds: readonly number[]) => { const posts = await this.postsService.findByUserIds([...userIds]); // IMPORTANT: return results in the SAME ORDER as input keys return userIds.map(id => posts.filter(p => p.authorId === id)); }, { cache: true }, // cache within the request lifetime ); } // Use in field resolver — each call queues the id @Resolver(() => User) export class UsersResolver { constructor(private postsLoader: PostsDataLoader) {} @ResolveField(() => [Post]) posts(@Parent() user: User): Promise { return this.postsLoader.batchLoadPosts.load(user.id); } } Scope.REQUEST — a new DataLoader instance per request; prevents cross-request cache contamination. Singleton DataLoader would serve cached data from previous requests to different users — a security issue. The batch function must return results in exactly the same order as the input keys array. Return null for missing keys — DataLoader expects the array length to match the keys length. cache: true (default) deduplicates repeated loads of the same key within one request.

Difficulty: 5/10

DataLoader integration, request-scoped providers, performance optimization

Implement DataLoader as an @Injectable({ scope: Scope.REQUEST }) service. Scope.REQUEST is critical — DataLoader caches keys for the lifetime of the request and a singleton loader would serve stale data across requests. The batch function receives an array of keys and must return results in the same order.

Request-scoped DataLoader service

DataLoader implementation rules:

1
Scope.REQUEST — a new DataLoader instance per request; prevents cross-request cache contamination.
2

How do you implement a DataLoader in NestJS and scope it correctly per request?

Difficulty: 5/10

DataLoader integration, request-scoped providers, performance optimization

Implement DataLoader as an @Injectable({ scope: Scope.REQUEST }) service. Scope.REQUEST is critical — DataLoader caches keys for the lifetime of the request and a singleton loader would serve stale data across requests. The batch function receives an array of keys and must return results in the same order.

Request-scoped DataLoader service

DataLoader implementation rules:

1
Scope.REQUEST — a new DataLoader instance per request; prevents cross-request cache contamination.
2

How do you implement a DataLoader in NestJS and scope it correctly per request?

How do you implement a DataLoader in NestJS and scope it correctly per request?

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