Quick Start

First install the module via yarn or npm and do not forget to install the database driver as well:

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

or

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

Next you will need to enable support for decorators as well as esModuleInterop in tsconfig.json via:

"experimentalDecorators": true,
"emitDecoratorMetadata": true,
"esModuleInterop": true

Then call MikroORM.init as part of bootstrapping your app:

To access driver specific methods like em.createQueryBuilder() we need to specify the driver type when calling MikroORM.init<D>(). Alternatively we can cast the orm.em to EntityManager exported from the driver package:

import { EntityManager } from '@mikro-orm/postgresql';
const em = orm.em as EntityManager;
const qb = em.createQueryBuilder(...);

import type { PostgreSqlDriver } from '@mikro-orm/postgresql'; // or any other driver package

const orm = await MikroORM.init<PostgreSqlDriver>({
  entities: ['./dist/entities'], // path to your JS entities (dist), relative to `baseDir`
  dbName: 'my-db-name',
  type: 'postgresql',
});
console.log(orm.em); // access EntityManager via `em` property

There are more ways to configure your entities, take a look at installation page.

Read more about all the possible configuration options in Advanced Configuration section.

Then you will need to fork entity manager for each request so their identity maps will not collide. To do so, use the RequestContext helper:

const app = express();

app.use((req, res, next) => {
  RequestContext.create(orm.em, next);
});

You should register this middleware as the last one just before request handlers and before any of your custom middleware that is using the ORM. There might be issues when you register it before request processing middleware like queryParser or bodyParser, so definitely register the context after them.

More info about RequestContext is described here.

Now you can start defining your entities (in one of the entities folders). This is how simple entity can look like in mongo driver:

./entities/MongoBook.ts

@Entity()
export class MongoBook {

  @PrimaryKey()
  _id: ObjectID;

  @SerializedPrimaryKey()
  id: string;

  @Property()
  title: string;

  @ManyToOne()
  author: Author;

  @ManyToMany()
  tags = new Collection<BookTag>(this);

  constructor(title: string, author: Author) {
    this.title = title;
    this.author = author;
  }

}

For SQL drivers, you can use id: number PK:

./entities/SqlBook.ts

@Entity()
export class SqlBook {

  @PrimaryKey()
  id: number;

}

Or if you want to use UUID primary keys:

./entities/UuidBook.ts

import { v4 } from 'uuid';

@Entity()
export class UuidBook {

  @PrimaryKey()
  uuid = v4();

}

More information can be found in defining entities section in docs.

When you have your entities defined, you can start using ORM either via EntityManager or via EntityRepositorys.

To save entity state to database, you need to persist it. Persist determines whether to use insert or update and computes appropriate change-set. Entity references that are not persisted yet (does not have identifier) will be cascade persisted automatically.

// use constructors in your entities for required parameters
const author = new Author('Jon Snow', 'snow@wall.st');
author.born = new Date();

const publisher = new Publisher('7K publisher');

const book1 = new Book('My Life on The Wall, part 1', author);
book1.publisher = publisher;
const book2 = new Book('My Life on The Wall, part 2', author);
book2.publisher = publisher;
const book3 = new Book('My Life on The Wall, part 3', author);
book3.publisher = publisher;

// just persist books, author and publisher will be automatically cascade persisted
await em.persistAndFlush([book1, book2, book3]);

To fetch entities from database you can use find() and findOne() of EntityManager:

const authors = em.find(Author, {});

for (const author of authors) {
  console.log(author); // instance of Author entity
  console.log(author.name); // Jon Snow

  for (const book of author.books) { // iterating books collection
    console.log(book); // instance of Book entity
    console.log(book.title); // My Life on The Wall, part 1/2/3
  }
}

More convenient way of fetching entities from database is by using EntityRepository, that carries the entity name so you do not have to pass it to every find and findOne calls:

const booksRepository = em.getRepository(Book);

const books = await booksRepository.find({ author: '...' }, {
  populate: ['author'],
  limit: 1,
  offset: 2,
  orderBy: { title: QueryOrder.DESC },
});

console.log(books); // Book[]

Take a look at docs about working with EntityManager or using EntityRepository instead.