NestJS InjectRepository() Guide with Examples

Overview

NestJS’s InjectRepository() is a decorator that simplifies the integration of the TypeORM package, making database operations in your application more efficient and type-safe. This guide elucidates its usage through incremental examples.

Understanding InjectRepository()

Before diving into practical examples, it is important to comprehend what InjectRepository() does. In NestJS, which adheres to the principles of Dependency Injection (DI), InjectRepository() is a custom decorator that allows you to inject a repository instance associated with a given entity directly into your service or provider. It is primarily used when you deal with TypeORM repositories.

Here is a quick look at the basic usage:

import { InjectRepository } from '@nestjs/typeorm';
import { YourEntity } from './your-entity.entity';
import { Repository } from 'typeorm';

export class YourService {
  constructor(
    @InjectRepository(YourEntity)
    private yourEntityRepository: Repository<YourEntity>,
  ) {}

  //... you can now use this.yourEntityRepository to interact with YourEntity's table in the database ...
}

Setting up a NestJS Project with TypeORM

To get started with InjectRepository(), set up a NestJS project with TypeORM support. Here’s how to create a new project and integrate TypeORM:

nest new project-name
# Navigate to your project directory
cd project-name
# Install TypeORM and the required database driver, e.g., PostgreSQL
npm install @nestjs/typeorm typeorm pg

Make sure to configure TypeORM in your application module:

import { Module } from '@nestjs/common';
import { TypeOrmModule } from '@nestjs/typeorm';

@Module({
  imports: [
    TypeOrmModule.forRoot({
      type: 'postgres', // the type of your database
      host: 'localhost',
      port: 5432,
      username: 'test',
      password: 'test',
      database: 'test',
      //... additional options ...
    }),
  ],
})
export class AppModule {}

Creating an Entity and Repository

After your project set-up, create an entity that TypeORM can link to a database table. This will demonstrate how to use InjectRepository().

import { Entity, PrimaryGeneratedColumn, Column } from 'typeorm';

@Entity()
export class Cat {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  //... additional columns ...
}

With an entity in place, let’s see how to inject the repository into a service.

import { Injectable } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository } from 'typeorm';
import { Cat } from './cat.entity';

@Injectable()
export class CatsService {
  constructor(
    @InjectRepository(Cat)
    private catsRepository: Repository<Cat>,
  ) {}

  findAll(): Promise<Cat[]> {
    return this.catsRepository.find();
  }

  // ... additional methods ...
}

Advanced Usage: Custom Repositories

Beyond basic entity repositories, you can define custom repositories to encapsulate specific sets of operations. Here’s how:

import { EntityRepository, Repository } from 'typeorm';
import { Cat } from './cat.entity';

@EntityRepository(Cat)
export class CatRepository extends Repository<Cat> {
  // Define custom methods here
}

Once defined, inject it the same way:

import { Injectable } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { CatRepository } from './cat.repository';

@Injectable()
export class CatsService {
  constructor(
    @InjectRepository(CatRepository)
    private catRepository: CatRepository,
  ) {}

  // Use your custom repository methods
}

Troubleshooting Common Issues

Working with InjectRepository() is generally straightforward, but you might encounter issues like circular dependencies or an incorrect provider scope. Take care to structure your modules and services to avoid such pitfalls and follow best practices to streamline the interaction with your database.

Conclusion

The InjectRepository() decorator is a cornerstone for many NestJS applications using TypeORM for database management. With the examples provided, you should have a solid foundation to build robust and maintainable services in your next NestJS project. Happy coding!