mikro-orm

What is mikro-orm?

Mikro-ORM is a TypeScript ORM for Node.js based on Data Mapper, Unit of Work, and Identity Map patterns. It supports multiple databases including MongoDB, MySQL, PostgreSQL, SQLite, and MariaDB. It provides a powerful and flexible way to interact with your database using TypeScript and JavaScript.

What are mikro-orm's main functionalities?

Entity Definition

Defines an entity with properties that map to database columns. The `@Entity` decorator marks the class as a database entity, while `@PrimaryKey` and `@Property` decorators define the primary key and other properties respectively.

```typescript
import { Entity, PrimaryKey, Property } from 'mikro-orm';

@Entity()
class User {
  @PrimaryKey()
  id!: number;

  @Property()
  name!: string;

  @Property()
  email!: string;
}
```

CRUD Operations

Demonstrates basic Create, Read, Update, and Delete (CRUD) operations. The `persistAndFlush` method saves a new entity to the database, and the `find` method retrieves entities from the database.

```typescript
const user = new User();
user.name = 'John Doe';
user.email = '[e-mail]';
await orm.em.persistAndFlush(user);

const users = await orm.em.find(User, {});
console.log(users);
```

Query Builder

Shows how to use the query builder to construct and execute complex queries. The `createQueryBuilder` method creates a new query builder instance, and the `select` and `where` methods build the query.

```typescript
const qb = orm.em.createQueryBuilder(User);
const users = await qb.select('*').where({ name: 'John Doe' }).execute();
console.log(users);
```

Transactions

Illustrates how to perform operations within a transaction. The `transactional` method ensures that all operations within the callback are executed within a single transaction.

```typescript
await orm.em.transactional(async em => {
  const user = new User();
  user.name = 'Jane Doe';
  user.email = '[e-mail]';
  await em.persistAndFlush(user);
});
```

Other packages similar to mikro-orm

typeorm

TypeORM is another TypeScript ORM for Node.js that supports Active Record and Data Mapper patterns. It is highly popular and supports multiple databases. Compared to Mikro-ORM, TypeORM has a larger community and more extensive documentation, but Mikro-ORM offers a more modern and flexible API.

sequelize

Sequelize is a promise-based Node.js ORM for Postgres, MySQL, MariaDB, SQLite, and Microsoft SQL Server. It is known for its simplicity and ease of use. While Sequelize is more mature and has a larger user base, Mikro-ORM provides better TypeScript support and more advanced features like Unit of Work and Identity Map patterns.

objection

Objection.js is an SQL-friendly ORM for Node.js, built on top of the SQL query builder Knex.js. It aims to stay as close to the SQL syntax as possible while providing a powerful and flexible API. Compared to Mikro-ORM, Objection.js offers more control over raw SQL queries but lacks some of the higher-level abstractions and features provided by Mikro-ORM.

README

mikro-orm

Simple typescript mongo ORM for node.js based on data-mapper, unit-of-work and identity-map patterns.

Heavily inspired by doctrine.

Installation & Usage

Fist install the module via yarn or npm:

$ yarn add mikro-orm

or

$ npm i -s mikro-orm

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

const orm = await MikroORM.init({
  entitiesDirs: ['entities'], // relative to `baseDir`
  dbName: 'my-db-name',
  clientUrl: '...', // defaults to 'mongodb://localhost:27017' 
  baseDir: __dirname, // defaults to `process.cwd()`
});
console.log(orm.em); // EntityManager

And do not forget to clear entity manager before each request if you do not want to store all loaded entities in memory:

const app = express();

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

Now you can define your entities (in one of the entitiesDirs folders):

Defining entity

@Entity({ collection: 'books-table' })
export class Book extends BaseEntity {

  @Property()
  title: string;

  @ManyToOne({ entity: () => Author.name })
  author: Author;

  @ManyToOne({ entity: () => Publisher.name })
  publisher: Publisher;

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

}

With your entities set up, you can start using entity manager and repositories as described in following section. For more examples, take a look at tests/EntityManager.test.ts.

Persisting and cascading

To save entity state to database, you need to persist it. Persist takes care or deciding 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', '[e-mail]');
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 orm.em.persist([book1, book2, book3]);

// or one by one
await orm.em.persist(book1, false);
await orm.em.persist(book2, false);
await orm.em.persist(book3); // flush everything to database at once

Fetching entities with EntityManager

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

API:

EntityManager.getCollection(entityName: string): Collection; // returns mongodb Collection for given entity
EntityManager.getRepository<T extends BaseEntity>(entityName: string): EntityRepository<T>;
EntityManager.find<T extends BaseEntity>(entityName: string, where?: FilterQuery<T>, populate?: string[], orderBy?: { [k: string]: 1 | -1; }, limit?: number, offset?: number): Promise<T[]>;
EntityManager.findOne<T extends BaseEntity>(entityName: string, where: FilterQuery<T> | string, populate?: string[]): Promise<T>;
EntityManager.merge<T extends BaseEntity>(entityName: string, data: any): T;
EntityManager.getReference<T extends BaseEntity>(entityName: string, id: string): T;
EntityManager.remove(entityName: string, where: BaseEntity | any): Promise<number>;
EntityManager.removeEntity(entity: BaseEntity): Promise<number>;
EntityManager.count(entityName: string, where: any): Promise<number>;
EntityManager.persist(entity: BaseEntity | BaseEntity[], flush?: boolean): Promise<void>;
EntityManager.flush(): Promise<void>;
EntityManager.clear(): void;
EntityManager.canPopulate(entityName: string, property: string): boolean;

Example:

const author = orm.em.findOne(Author.name, '...id...');
const books = orm.em.find(Book.name, {});

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

  for (const book of author.books) {
    console.log(book.title); // initialized
    console.log(book.author.isInitialized()); // true
    console.log(book.author.id);
    console.log(book.author.name); // Jon Snow
    console.log(book.publisher); // just reference
    console.log(book.publisher.isInitialized()); // false
    console.log(book.publisher.id);
    console.log(book.publisher.name); // undefined
  }
}

Using EntityRepository instead of EntityManager

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:

API:

EntityRepository.persist(entity: BaseEntity, flush?: boolean): Promise<void>;
EntityRepository.findOne(where: FilterQuery<BaseEntity> | string, populate?: string[]): Promise<BaseEntity>;
EntityRepository.find(where: FilterQuery<BaseEntity>, populate?: string[], orderBy?: { [k: string]: 1 | -1; }, limit?: number, offset?: number): Promise<BaseEntity[]>;
EntityRepository.findAll(populate?: string[], orderBy?: { [k: string]: 1 | -1; }, limit?: number, offset?: number): Promise<BaseEntity[]>;
EntityRepository.remove(where: BaseEntity | any): Promise<number>;
EntityRepository.flush(): Promise<void>;
EntityRepository.canPopulate(property: string): boolean;
EntityRepository.count(where?: any): Promise<number>;

Example:

const booksRepository = orm.em.getRepository<Book>(Book.name);

// with sorting, limit and offset parameters, populating author references
const books = await booksRepository.find({ author: '...' }, ['author'], { title: -1 }, 2, 1);
console.log(books); // Book[]

Custom repository

To use custom repository, just extend EntityRepository<T> class:

export class CustomAuthorRepository extends EntityRepository<Author> {

  // your custom methods...
  public findAndUpdate(...) {
    // ...
  }

}

And register your repository as @Entity decorator:

@Entity({ customRepository: CustomAuthorRepository })
export class Publisher extends BaseEntity {
  // ...
}

Then your custom repository can be accessed via EntityManager.getRepository() method.

Core features

Identity Map

MikroORM uses identity map in background so you will always get the same instance of one entity.

const authorRepository = orm.em.getRepository<Author>(Author.name);
const jon = await authorRepository.findOne({ name: 'Jon Snow' }, ['books']);
const authors = await authorRepository.findAll(['books']);

// identity map in action
console.log(jon === authors[0]); // true

If you want to clear this identity map cache, you can do so via EntityManager.clear() method:

orm.em.clear();

Entity references

Every single entity relation is mapped to an entity reference. Reference is an entity that has only its identifier. This reference is stored in identity map so you will get the same object reference when fetching the same document from database.

You can call await entity.init() to initialize the entity. This will trigger database call and populate itself, keeping the same reference in identity map.

const author = await orm.em.getReference('...id...');
console.log(author.id); // accessing the id will not trigger any db call
console.log(author.isInitialized()); // false
console.log(author.name); // undefined

await author.init(); // this will trigger db call
console.log(author.isInitialized()); // true
console.log(author.name); // defined

Using entity constructors

Internally, MikroORM never calls entity constructor, so you are free to use it as you wish. The constructor will be called only when you instantiate the class yourself via new operator, so it is a handy place to require your data when creating new entity.

For example following Book entity definition will always require to set title and author, but publisher will be optional:

@Entity()
export class Book extends BaseEntity {

  @Property()
  title: string;

  @ManyToOne({ entity: () => Author.name })
  author: Author;

  @ManyToOne({ entity: () => Publisher.name })
  publisher: Publisher;

  @ManyToMany({ entity: () => BookTag.name, inversedBy: 'books' })
  tags: Collection<BookTag>;

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

}

ObjectID and string duality

Every entity has both ObjectID and string id available, also all methods of EntityManager and EntityRepository supports querying by both of them.

const author = await orm.em.getReference('...id...');
console.log(author.id);  // returns '...id...'
console.log(author._id); // returns ObjectID('...id...')

// all of those will return the same results
const article = '...article id...'; // string id
const book = '...book id...'; // string id
const repo = orm.em.getRepository<Author>(Author.name);
const foo1 = await repo.find({ id: { $in: [article] }, favouriteBook: book });
const bar1 = await repo.find({ id: { $in: [new ObjectID(article)] }, favouriteBook: new ObjectID(book) });
const foo2 = await repo.find({ _id: { $in: [article] }, favouriteBook: book });
const bar2 = await repo.find({ _id: { $in: [new ObjectID(article)] }, favouriteBook: new ObjectID(book) });

Collections

OneToMany and ManyToMany collections are stored in a Collection wrapper. It implements iterator so you can use for of loop to iterate through it.

const author = orm.em.findOne(Author.name, '...', ['books']); // populating books collection

// or we could lazy load books collection later via `init()` method
await author.books.init();

for (const book of author.books) {
  console.log(book.title); // initialized
  console.log(book.author.isInitialized()); // true
  console.log(book.author.id);
  console.log(book.author.name); // Jon Snow
  console.log(book.publisher); // just reference
  console.log(book.publisher.isInitialized()); // false
  console.log(book.publisher.id);
  console.log(book.publisher.name); // undefined
}

// collection needs to be initialized before you can work with it
author.books.add(book);
console.log(author.books.contains(book)); // true
author.books.remove(book);
console.log(author.books.contains(book)); // false
author.books.add(book);
console.log(author.books.count()); // 1
author.books.removeAll();
console.log(author.books.contains(book)); // false
console.log(author.books.count()); // 0
console.log(author.books.getItems()); // 0
console.log(author.books.getIdentifiers()); // array of ObjectID
console.log(author.books.getIdentifiers('id')); // array of string

OneToMany collections

OneToMany collections are inverse side of ManyToOne references, to which they need to point via fk attribute:

@Entity()
export class Book extends BaseEntity {

  @ManyToOne({ entity: () => Author.name })
  author: Author;

}

@Entity()
export class BookTag extends BaseEntity {

  @OneToMany({ entity: () => Book.name, fk: 'author' })
  books: Collection<Book>;

}

ManyToMany collections

As opposed to SQL databases, with MongoDB we do not need to have join tables for ManyToMany relations. All references are stored as an array of ObjectIDs on owning entity.

Unidirectional

Unidirectional ManyToMany relations are defined only on one side, and marked explicitly as owner:

@ManyToMany({ entity: () => Book.name, owner: true })
books: Collection<Book>;

Bidirectional

Bidirectional ManyToMany relations are defined on both sides, while one is owning side (where references are store), marked by inversedBy attribute pointing to the inverse side:

@ManyToMany({ entity: () => BookTag.name, inversedBy: 'books' })
tags: Collection<BookTag>;

And on the inversed side we define it with mappedBy attribute poining back to the owner:

@ManyToMany({ entity: () => Book.name, mappedBy: 'tags' })
books: Collection<Book>;

Updating entity values with BaseEntity.assign()

When you want to update entity based on user input, you will usually have just plain string ids of entity relations as user input. Normally you would need to use EntityManager.getReference() to create references from each id first, and then use those references to update entity relations:

const jon = new Author('Jon Snow', '[e-mail]');
const book = new Book('Book', jon);
book.author = orm.em.getReference<Author>(Author.name, '...id...');

Same result can be easily achieved with BaseEntity.assign():

book.assign({ 
  title: 'Better Book 1', 
  author: '...id...',
});
console.log(book.title); // 'Better Book 1'
console.log(book.author); // instance of Author with id: '...id...'
console.log(book.author.id); // '...id...'

TODO

• cascade persist in collections
• aggregate support?
• improve populating of collections in EM#find() method
• add nativeUpdate and nativeDelete (without hooks support), allow only entities in EM#remove
• cascade remove references on other entities when deleting entity (e.g. from M:N collection)

TODO docs

• lifecycle hooks
• property type validation