Using Data Loaders for Efficient GraphQL Queries in NestJS

Introduction

When building GraphQL APIs, one common problem developers face is the “N+1 query problem.” This occurs when a GraphQL query results in multiple database queries, leading to inefficiencies and slow performance. A tool called DataLoader can help solve this problem by batching and caching database requests. In this article, we’ll learn how to use DataLoader in a NestJS GraphQL application to make our queries more efficient.

What is DataLoader?

DataLoader is a utility that batches and caches requests to a database or any other data source. It helps avoid the N+1 problem by grouping multiple requests into a single batch, reducing the number of queries made to the database.

Setting Up a NestJS Project

First, let’s create a new NestJS project:

npm i -g @nestjs/cli
nest new dataloader-example
cd dataloader-example

Next, install the necessary GraphQL and DataLoader packages:

npm install @nestjs/graphql @nestjs/apollo apollo-server-express graphql

npm install dataloader

Creating the DataLoader:

// user.dataloader.ts
import * as DataLoader from 'dataloader';
import { Injectable, Scope } from '@nestjs/common';
import { UserService } from './user.service';
import { User } from './user.entity';

@Injectable({ scope: Scope.REQUEST })
export class UserDataLoader {
  constructor(private userService: UserService) {}

  createLoader() {
    return new DataLoader<number, User[]>(async (userIds: number[]) => {
      const users = await Promise.all(userIds.map(id => this.userService.findFriends(id)));
      return userIds.map(id => users.find(user => user.id === id).friends);
    });
  }
}

Injecting DataLoader in the Resolver:

// user.resolver.ts
import { Resolver, Query, Args, ResolveField, Parent, Context } from '@nestjs/graphql';
import { User } from './user.entity';
import { UserService } from './user.service';
import { UserDataLoader } from './user.dataloader';

@Resolver(of => User)
export class UserResolver {
  constructor(
    private userService: UserService,
    private userDataLoader: UserDataLoader,
  ) {}

  @Query(returns => User)
  async user(@Args('id') id: number): Promise<User> {
    return this.userService.findById(id);
  }

  @ResolveField()
  async friends(@Parent() user: User, @Context() context: any): Promise<User[]> {
    const loader = context.userDataLoader || this.userDataLoader.createLoader();
    return loader.load(user.id);
  }
}

roviding DataLoader in the GraphQL Context:

// app.module.ts
import { Module } from '@nestjs/common';
import { GraphQLModule } from '@nestjs/graphql';
import { UserModule } from './user/user.module';
import { UserDataLoader } from './user/user.dataloader';

@Module({
  imports: [
    GraphQLModule.forRoot({
      autoSchemaFile: true,
      context: ({ req }) => ({
        userDataLoader: new UserDataLoader().createLoader(),
      }),
    }),
    UserModule,
  ],
  providers: [UserDataLoader],
})
export class AppModule {}

Conclusion

By using DataLoader in our NestJS GraphQL application, we can optimize our database queries, reducing the number of requests and improving performance. This approach helps solve the N+1 query problem, ensuring that our GraphQL API remains efficient and scalable.