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    5.6.8 • Public • Published


    TypeScript ORM for Node.js based on Data Mapper, Unit of Work and Identity Map patterns. Supports MongoDB, MySQL, MariaDB, PostgreSQL and SQLite databases.

    Heavily inspired by Doctrine and Nextras Orm.

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    🤔 Unit of What?

    You might be asking: What the hell is Unit of Work and why should I care about it?

    Unit of Work maintains a list of objects (entities) affected by a business transaction and coordinates the writing out of changes. (Martin Fowler)

    Identity Map ensures that each object (entity) gets loaded only once by keeping every loaded object in a map. Looks up objects using the map when referring to them. (Martin Fowler)

    So what benefits does it bring to us?

    Implicit Transactions

    First and most important implication of having Unit of Work is that it allows handling transactions automatically.

    When you call em.flush(), all computed changes are queried inside a database transaction (if supported by given driver). This means that you can control the boundaries of transactions simply by calling em.persistLater() and once all your changes are ready, calling flush() will run them inside a transaction.

    You can also control the transaction boundaries manually via em.transactional(cb).

    const user = await em.findOneOrFail(User, 1);
    user.email = 'foo@bar.com';
    const car = new Car();
    // thanks to bi-directional cascading we only need to persist user entity
    // flushing will create a transaction, insert new car and update user with new email
    // as user entity is managed, calling flush() is enough
    await em.flush();

    ChangeSet based persistence

    MikroORM allows you to implement your domain/business logic directly in the entities. To maintain always valid entities, you can use constructors to mark required properties. Let's define the User entity used in previous example:

    export class User {
      id!: number;
      name!: string;
      address?: Address;
      cars = new Collection<Car>(this);
      constructor(name: string) {
        this.name = name;

    Now to create new instance of the User entity, we are forced to provide the name:

    const user = new User('John Doe'); // name is required to create new user instance
    user.address = new Address('10 Downing Street'); // address is optional

    Once your entities are loaded, make a number of synchronous actions on your entities, then call em.flush(). This will trigger computing of change sets. Only entities (and properties) that were changed will generate database queries, if there are no changes, no transaction will be started.

    const user = await em.findOneOrFail(User, 1, ['cars', 'address']);
    user.title = 'Mr.';
    user.address.street = '10 Downing Street'; // address is 1:1 relation of Address entity
    user.cars.getItems().forEach(car => car.forSale = true); // cars is 1:m collection of Car entities
    const car = new Car('VW');
    // now we can flush all changes done to managed entities
    await em.flush();

    em.flush() will then execute these queries from the example above:

    update user set title = 'Mr.' where id = 1;
    update user_address set street = '10 Downing Street' where id = 123;
    update car set for_sale = true where id = 1;
    update car set for_sale = true where id = 2;
    update car set for_sale = true where id = 3;
    insert into car (brand, owner) values ('VW', 1);

    Only One Instance of Entity

    Thanks to Identity Map, you will always have only one instance of given entity in one context. This allows for some optimizations (skipping loading of already loaded entities), as well as comparison by identity (ent1 === ent2).

    📖 Documentation

    MikroORM v4 documentation, included in this repo in the root directory, is built with Jekyll and publicly hosted on GitHub Pages at https://mikro-orm.io.

    There is also auto-generated CHANGELOG.md file based on commit messages (via semantic-release).

    You can browse MikroORM v3 docs at https://mikro-orm.io/docs/3.6/installation.

    To upgrade to v4, please see the upgrading guide.

    Core Features

    📦 Example Integrations

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

    TypeScript Examples

    JavaScript Examples


    • Introducing MikroORM, TypeScript data-mapper ORM with Identity Map
    • Handling transactions and concurrency in MikroORM
    • MikroORM 3: Knex.js, CLI, Schema Updates, Entity Generator and more…

    🚀 Quick Start

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

    Since v4, you should install the driver package, but not the db connector itself, e.g. install @mikro-orm/sqlite, but not sqlite3 as that is already included in the driver package.

    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


    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(). 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 SQL 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:


    export class MongoBook {
      _id: ObjectID;
      id: string;
      title: string;
      author: Author;
      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:


    export class SqlBook {
      id: number;

    Or if you want to use UUID primary keys:


    import { v4 } from 'uuid';
    export class UuidBook {
      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 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', '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 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,
      orderBy: { title: QueryOrder.DESC },
    console.log(books); // Book[]

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

    🤝 Contributing

    Contributions, issues and feature requests are welcome. Please read CONTRIBUTING.md for details on the process for submitting pull requests to us.


    👤 Martin Adámek

    See also the list of contributors who participated in this project.

    Show Your Support

    Please ⭐️ this repository if this project helped you!

    📝 License

    Copyright © 2018 Martin Adámek.

    This project is licensed under the MIT License - see the LICENSE file for details.


    npm i mikro-orm



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