Using MikroORM with NestJS framework

Installation

Easiest way to integrate MikroORM to Nest is via @mikro-orm/nestjs module. Simply install it next to Nest, MikroORM and underlying driver:

$ yarn add @mikro-orm/core @mikro-orm/nestjs @mikro-orm/mongodb     # for mongo
$ yarn add @mikro-orm/core @mikro-orm/nestjs @mikro-orm/mysql       # for mysql/mariadb
$ yarn add @mikro-orm/core @mikro-orm/nestjs @mikro-orm/mariadb     # for mysql/mariadb
$ yarn add @mikro-orm/core @mikro-orm/nestjs @mikro-orm/postgresql  # for postgresql
$ yarn add @mikro-orm/core @mikro-orm/nestjs @mikro-orm/sqlite      # for sqlite

or

$ npm i -s @mikro-orm/core @mikro-orm/nestjs @mikro-orm/mongodb     # for mongo
$ npm i -s @mikro-orm/core @mikro-orm/nestjs @mikro-orm/mysql       # for mysql/mariadb
$ npm i -s @mikro-orm/core @mikro-orm/nestjs @mikro-orm/mariadb     # for mysql/mariadb
$ npm i -s @mikro-orm/core @mikro-orm/nestjs @mikro-orm/postgresql  # for postgresql
$ npm i -s @mikro-orm/core @mikro-orm/nestjs @mikro-orm/sqlite      # for sqlite

Once the installation process is completed, we can import the MikroOrmModule into the root AppModule.

@Module({
  imports: [
    MikroOrmModule.forRoot({
      entities: ['./dist/entities'],
      entitiesTs: ['./src/entities'],
      dbName: 'my-db-name.sqlite3',
    }),
  ],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

The forRoot() method accepts the same configuration object as init() from the MikroORM package. Check this page for the complete configuration documentation.

Alternatively we can configure the CLI by creating a configuration file mikro-orm.config.ts and then call the forRoot() without any arguments. This won't work when you use a build tools that use tree shaking.

@Module({
  imports: [
    MikroOrmModule.forRoot(),
  ],
  ...
})
export class AppModule {}

Afterward, the EntityManager will be available to inject across entire project (without importing any module elsewhere).

import { MikroORM } from '@mikro-orm/core';
import { EntityManager } from '@mikro-orm/mysql'; // Import EntityManager from your driver package or `@mikro-orm/knex`

@Injectable()
export class MyService {

  constructor(private readonly orm: MikroORM,
              private readonly em: EntityManager) {
  }

}

Notice that the EntityManager is imported from the @mikro-orm/driver package, where driver is mysql, sqlite, postgres or whatever driver you are using.

In case you have @mikro-orm/knex installed as a dependency, you can also import the EntityManager from there.

Repositories

MikroORM supports the repository design pattern. For every entity we can create a repository. Read the complete documentation on repositories here. To define which repositories shall be registered in the current scope you can use the forFeature() method. For example, in this way:

You should not register your base entities via forFeature(), as there are no repositories for those. On the other hand, base entities need to be part of the list in forRoot() (or in the ORM config in general).

// photo.module.ts

@Module({
  imports: [MikroOrmModule.forFeature([Photo])],
  providers: [PhotoService],
  controllers: [PhotoController],
})
export class PhotoModule {}

and import it into the root AppModule:

// app.module.ts
@Module({
  imports: [MikroOrmModule.forRoot(...), PhotoModule],
})
export class AppModule {}

In this way we can inject the PhotoRepository to the PhotoService using the @InjectRepository() decorator:

@Injectable()
export class PhotoService {
  constructor(
    @InjectRepository(Photo)
    private readonly photoRepository: EntityRepository<Photo>
  ) {}

  // ...
}

Using custom repositories

When using custom repositories, we can get around the need for @InjectRepository() decorator by naming our repositories the same way as getRepositoryToken() method do:

export const getRepositoryToken = <T> (entity: EntityName<T>) => `${Utils.className(entity)}Repository`;

In other words, as long as we name the repository same was as the entity is called, appending Repository suffix, the repository will be registered automatically in the Nest.js DI container.

**./author.entity.ts**

@Entity({ repository: () => AuthorRepository })
export class Author {

  // to allow inference in `em.getRepository()`
  [EntityRepositoryType]?: AuthorRepository;

}

**./author.repository.ts**

import { EntityRepository } from '@mikro-orm/mysql'; // Import EntityManager from your driver package or `@mikro-orm/knex`

export class AuthorRepository extends EntityRepository<Author> {

  // your custom methods...

}

As the custom repository name is the same as what getRepositoryToken() would return, we do not need the @InjectRepository() decorator anymore:

@Injectable()
export class MyService {

  constructor(private readonly repo: AuthorRepository) { }

}

Load entities automatically

autoLoadEntities option was added in v4.1.0

Manually adding entities to the entities array of the connection options can be tedious. In addition, referencing entities from the root module breaks application domain boundaries and causes leaking implementation details to other parts of the application. To solve this issue, static glob paths can be used.

Note, however, that glob paths are not supported by webpack, so if you are building your application within a monorepo, you won't be able to use them. To address this issue, an alternative solution is provided. To automatically load entities, set the autoLoadEntities property of the configuration object (passed into the forRoot() method) to true, as shown below:

@Module({
  imports: [
    MikroOrmModule.forRoot({
      // ...
      autoLoadEntities: true,
    }),
  ],
})
export class AppModule {}

With that option specified, every entity registered through the forFeature() method will be automatically added to the entities array of the configuration object.

Note that entities that aren't registered through the forFeature() method, but are only referenced from the entity (via a relationship), won't be included by way of the autoLoadEntities setting.

Using autoLoadEntities also has no effect on the MikroORM CLI - for that we still need CLI config with the full list of entities. On the other hand, we can use globs there, as the CLI won't go through webpack.

Request scoped handlers in queues

@CreateRequestContext() decorator is available in @mikro-orm/core package.

Before v6, @CreateRequestContext() was called @UseRequestContext().

As mentioned in the docs, we need a clean state for each request. That is handled automatically thanks to the RequestContext helper registered via middleware.

But middlewares are executed only for regular HTTP request handles, what if we need a request scoped method outside of that? One example of that is queue handlers or scheduled tasks.

We can use the @CreateRequestContext() decorator. It requires you to first inject the MikroORM instance to current context, it will be then used to create the context for you. Under the hood, the decorator will register new request context for your method and execute it inside the context.

@CreateRequestContext() should be used only on the top level methods. It should not be nested - a method decorated with it should not call another method that is also decorated with it.

@Controller()
export class MyService {

  constructor(private readonly orm: MikroORM) { }

  @CreateRequestContext()
  async doSomething() {
    // this will be executed in a separate context
  }

}

Alternatively you can provide a callback that will return the MikroORM instance.

import { DI } from '..';

export class MyService {

  @CreateRequestContext(() => DI.orm)
  async doSomething() {
    // this will be executed in a separate context
  }

}

Another thing to look out for how you combine them with other decorators. For example if you use it in combination with NestJS's "BullJS queues module", a safe bet is to extract the part of the code that needs a clean docs, either in a new method or inject a separate service.

@Processor({
  name: 'example-queue',
})
export class MyConsumer {
  constructor(private readonly orm: MikroORM) { }

  @Process()
  async doSomething(job: Job<any>) {
    await this.doSomethingWithMikro();
  }

  @CreateRequestContext()
  async doSomethingWithMikro() {
    // this will be executed in a separate context
  }
}

As in this case, the @Process() decorator expects to receive an executable function, but if we add @CreateRequestContext() to the handler as well, if @CreateRequestContext() is executed before @Process(), the later will receive void.

@EnsureRequestContext() decorator

Sometimes you may prefer to just ensure the method is executed inside a request context, and reuse the existing context if available. You can use the @EnsureRequestContext() decorator here, it behaves exactly like the @CreateRequestContext, but only creates new context if necessary, reusing the existing one if possible.

Request scoping when using GraphQL

The GraphQL module in NestJS uses apollo-server-express which enables bodyparser by default. (source) As mentioned in "RequestContext helper for DI containers" this causes issues as the Middleware the NestJS MikroORM module installs needs to be loaded after bodyparser. At the same time make sure to disable body-parser in NestJs itself as well.

This can be done by adding bodyparser to your main.ts file

import { NestFactory } from '@nestjs/core';
import express from 'express';
async function bootstrap() {
  const app = await NestFactory.create(AppModule,{ bodyParser: false });
  app.use(express.json());
  await app.listen(5555);
}

And at the same time disabling the bodyparser in the GraphQL Module

@Module({
  imports: [
    GraphQLModule.forRoot({
      bodyParserConfig: false,
    }),
  ],
})

App shutdown and cleanup

By default, NestJS does not listen for system process termination signals (for example SIGTERM). Because of this, the MikroORM shutdown logic will never execute if the process is terminated, which could lead to database connections remaining open and consuming resources. To enable this, the enableShutdownHooks function needs to be called when starting up the application.

async function bootstrap() {
  const app = await NestFactory.create(AppModule);

  // Starts listening for shutdown hooks
  app.enableShutdownHooks();

  await app.listen(3000);
}

More information about enableShutdownHooks

Multiple Database Connections

You can define multiple database connections by registering multiple MikroOrmModule and setting their contextName. If you want to use middleware request context you must disable automatic middleware and register MikroOrmModule with forMiddleware() or use NestJS Injection Scope

@Module({
  imports: [
    MikroOrmModule.forRoot({
      contextName: 'db1',
      registerRequestContext: false, // disable automatatic middleware
      ...
    }),
    MikroOrmModule.forRoot({
      contextName: 'db2',
      registerRequestContext: false, // disable automatatic middleware
      ...
    }),
    MikroOrmModule.forMiddleware()
  ],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

To access different MikroORM/EntityManager connections you have to use the new injection tokens @InjectMikroORM()/@InjectEntityManager() where you are required to pass the contextName in:

@Injectable()
export class MyService {

  constructor(@InjectMikroORM('db1') private readonly orm1: MikroORM,
              @InjectMikroORM('db2') private readonly orm2: MikroORM,
              @InjectEntityManager('db1') private readonly em1: EntityManager,
              @InjectEntityManager('db2') private readonly em2: EntityManager) { }

}

When defining your repositories with forFeature() method you will need to set which contextName you want it registered against:

// photo.module.ts

@Module({
  imports: [MikroOrmModule.forFeature([Photo], 'db1')],
  providers: [PhotoService],
  controllers: [PhotoController],
})
export class PhotoModule {}

When using the @InjectRepository decorator you will also need to pass the contextName you want to get it from:

@Injectable()
export class PhotoService {
  constructor(
    @InjectRepository(Photo, 'db1')
    private readonly photoRepository: EntityRepository<Photo>
  ) {}

  // ...

}

Serialization caveat

NestJS built-in serialization relies on class-transformer. Since MikroORM wraps every single entity relation in a Reference or a Collection instance (for type-safety), this will make the built-in ClassSerializerInterceptor blind to any wrapped relations. In other words, if you return MikroORM entities from your HTTP or WebSocket handlers, all of their relations will NOT be serialized.

Luckily, MikroORM provides a serialization API which can be used in lieu of ClassSerializerInterceptor.

@Entity()
export class Book {
  @Property({ hidden: true })   // --> Equivalent of class-transformer's `@Exclude`
  hiddenField: number = Date.now();

  @Property({ persist: false }) // --> Will only exist in memory (and will be serialized). Similar to class-transformer's `@Expose()`
  count?: number;

  @ManyToOne({ serializer: value => value.name, serializedName: 'authorName' })   // Equivalent of class-transformer's `@Transform()`
  author: Author;
}

Testing

The @mikro-orm/nestjs package exposes getRepositoryToken() function that returns prepared token based on a given entity to allow mocking the repository.

If we register the provider only via getRepositoryToken(), we need to use the @InjectRepository decorator. To be able to use custom repository without this decorator, we need to register it with provide: PhotoRepository. provide:

@Module({
  providers: [
    PhotoService,
    // required for `@InjectRepository` decorator
    {
      provide: getRepositoryToken(Photo),
      useValue: mockedRepository,
    },

    // required for custom repositories if we don't want to use `@InjectRepository`
    {
      provide: PhotoRepository,
      useValue: mockedRepository,
    },
  ],
})
export class PhotoModule {}

Using EventSubscriber

Subscribers are normally registered only via the ORM config, but you can do so also dynamically via EventManager.registerSubscriber(). If you want your subscriber to use some dependencies from the Nest.js DI container, use the @Injectable decorator and register the subscriber manually in the constructor instead of passing it to the ORM config:

import { Injectable } from '@nestjs/common';
import { EntityName, EventArgs, EventSubscriber } from '@mikro-orm/core';

@Injectable()
export class AuthorSubscriber implements EventSubscriber<Author> {

  constructor(em: EntityManager) {
    em.getEventManager().registerSubscriber(this);
  }

  getSubscribedEntities(): EntityName<Author>[] {
    return [Author];
  }

  async afterCreate(args: EventArgs<Author>): Promise<void> {
    // ...
  }

  async afterUpdate(args: EventArgs<Author>): Promise<void> {
    // ...
  }

}

GraphQL resolvers

MikroORM supports dataloaders natively since v6 for Reference.load() and Collection.load() methods.

@Resolver(() => Book)
class BookResolver {

  @ResolveField(() => Author)
  async author(@Parent() book: Book) {
    return book.author.load({ dataloader: true }); // can be also enabled globally
  }

}

Example

A real world example of NestJS with MikroORM can be found here