typescript 36 lines · 7 steps

Batching queries with DataLoader in NestJS

A request-scoped provider that collapses many user lookups into a single batched database query.

Explained by highlit
NestJS Intermediate dataloader batching n+1 dependency-injection caching
1import { Injectable, Scope } from '@nestjs/common';
2import DataLoader from 'dataloader';
3import { InjectRepository } from '@nestjs/typeorm';
4import { In, Repository } from 'typeorm';
5import { User } from './entities/user.entity';
6 
7@Injectable({ scope: Scope.REQUEST })
8export class UserLoader {
9 private readonly byId: DataLoader<string, User>;
10 
11 constructor(
12 @InjectRepository(User)
13 private readonly users: Repository<User>,
14 ) {
15 this.byId = new DataLoader<string, User>(
16 async (ids) => this.batchLoad(ids),
17 { maxBatchSize: 100 },
18 );
19 }
20 
21 load(id: string): Promise<User> {
22 return this.byId.load(id);
23 }
24 
25 loadMany(ids: readonly string[]): Promise<(User | Error)[]> {
26 return this.byId.loadMany(ids);
27 }
28 
29 private async batchLoad(ids: readonly string[]): Promise<(User | Error)[]> {
30 const rows = await this.users.findBy({ id: In([...ids]) });
31 const map = new Map(rows.map((row) => [row.id, row]));
32 return ids.map(
33 (id) => map.get(id) ?? new Error(`User ${id} not found`),
34 );
35 }
36}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1DataLoader coalesces individual lookups fired in the same tick into one batched query, killing N+1 patterns.
  2. 2The batch function must return results in the exact order of the requested keys, filling gaps explicitly.
  3. 3Request scope gives each request its own loader instance, so DataLoader's cache never leaks between users.

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