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

Simple typescript ORM for node.js based on data-mapper, unit-of-work and identity-map patterns. Supports MongoDB, MySQL, PostgreSQL and SQLite databases.

Heavily inspired by Doctrine and Nextras Orm.

Documentation

MikroORM's documentation, included in this repo in the root directory, is built with Jekyll and publicly hosted on GitHub Pages at https://b4nan.github.io/mikro-orm/.

Core features

• Clean and simple entity definition
• Identity Map
• Entity references
• Using entity constructors
• Collections
• Cascading persist and remove
• Preloading deeply nested structures via populate
• Property validation
• Lifecycle hooks
• Vanilla JS support

Example integrations

You can find example integrations for some popular frameworks in the mikro-orm-examples repository:

TypeScript examples

• Express + MongoDB
• Nest + MySQL

JavaScript examples

• Express + MongoDB

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 mongodb # for mongo
$ yarn add mikro-orm mysql2  # for mysql
$ yarn add mikro-orm pg      # for postgresql
$ yarn add mikro-orm sqlite  # for sqlite

or

$ npm i -s mikro-orm mongodb # for mongo
$ npm i -s mikro-orm mysql2  # for mysql
$ npm i -s mikro-orm pg      # for postgresql
$ npm i -s mikro-orm sqlite  # for sqlite

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

"experimentalDecorators": true

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

const orm = await MikroORM.init({
  entitiesDirs: ['./dist/entities'], // path to your JS entities (dist), relative to `baseDir`
  dbName: 'my-db-name',
  clientUrl: '...', // defaults to 'mongodb://localhost:27017' for mongodb driver
});
console.log(orm.em); // access EntityManager via `em` property

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

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 entitiesDirs folders):

./entities/Book.ts

@Entity()
export class Book {

  @PrimaryKey()
  _id: ObjectID;

  @Property()
  title: string;

  @ManyToOne()
  author: Author;

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

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

}

export interface Book extends IEntity { }

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 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.persistAndFlush([book1, book2, book3]);

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

const authors = orm.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 = orm.em.getRepository(Book);

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

// or with options object
const books = await booksRepository.find({ author: '...' }, { 
  populate: ['author'],
  limit: 1,
  offset: 2,
  sort: { title: QueryOrder.DESC },
});

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

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