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Version: 3.6

Installation & Usage

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/mariadb
$ yarn add mikro-orm mariadb # for mysql/mariadb
$ yarn add mikro-orm pg # for postgresql
$ yarn add mikro-orm sqlite3 # for sqlite


$ npm i -s mikro-orm mongodb # for mongo
$ npm i -s mikro-orm mysql2 # for mysql/mariadb
$ npm i -s mikro-orm mariadb # for mysql/mariadb
$ npm i -s mikro-orm pg # for postgresql
$ npm i -s mikro-orm sqlite3 # for sqlite

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

"experimentalDecorators": true,
"esModuleInterop": true

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

const orm = await MikroORM.init({
entities: [Author, Book, BookTag],
dbName: 'my-db-name',
type: 'mongo', // one of `mongo` | `mysql` | `mariadb` | `postgresql` | `sqlite`
clientUrl: '...', // defaults to 'mongodb://localhost:27017' for mongodb driver
console.log(orm.em); // access EntityManager via `em` property

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

You can also provide paths where you store your entities via entitiesDirs array. Internally it uses globby so you can use globbing patterns.

const orm = await MikroORM.init({
entitiesDirs: ['./dist/app/**/entities'],
// ...

You should provide list of directories, not paths to entities directly. If you want to do that instead, you should use entities array and use globby manually:

import globby from 'globby';

const orm = await MikroORM.init({
entities: await (globby('./dist/app/**/entities/*.js')).map(require),
// ...

You can pass additional options to the underlying driver (e.g. mysql2) via driverOptions. The object will be deeply merged, overriding all internally used options.

Possible issues with circular dependencies

Your entities will most probably contain circular dependencies (e.g. if you use bi-directional relationship). While this is fine, there might be issues caused by wrong order of entities during discovery, especially when you are using the folder based way (via entitiesDirs).

The errors caused by circular dependencies are usually similar to this one:

TypeError: Cannot read property 'name' of undefined
at Function.className (/path/to/project/node_modules/mikro-orm/dist/utils/Utils.js:253:28)
at TsMorphMetadataProvider.extractType (/path/to/project/node_modules/mikro-orm/dist/metadata/TsMorphMetadataProvider.js:37:34)
at TsMorphMetadataProvider.initProperties (/path/to/project/node_modules/mikro-orm/dist/metadata/TsMorphMetadataProvider.js:25:31)
at TsMorphMetadataProvider.loadEntityMetadata (/path/to/project/node_modules/mikro-orm/dist/metadata/TsMorphMetadataProvider.js:16:9)
at MetadataDiscovery.discoverEntity (/path/to/project/node_modules/mikro-orm/dist/metadata/MetadataDiscovery.js:109:9)
at MetadataDiscovery.discoverDirectory (/path/to/project/node_modules/mikro-orm/dist/metadata/MetadataDiscovery.js:80:13)
at Function.runSerial (/path/to/project/node_modules/mikro-orm/dist/utils/Utils.js:303:22)
at MetadataDiscovery.findEntities (/path/to/project/node_modules/mikro-orm/dist/metadata/MetadataDiscovery.js:56:13)
at (/path/to/project/node_modules/mikro-orm/dist/metadata/MetadataDiscovery.js:30:9)
at Function.init (/path/to/project/node_modules/mikro-orm/dist/MikroORM.js:45:24)
at Function.handleSchemaCommand (/path/to/project/node_modules/mikro-orm/dist/cli/SchemaCommandFactory.js:51:21)

If you encounter this, you have basically two options:

  • Use entities array to have control over the order of discovery. You might need to play with the actual order you provide here, or possibly with the order of import statements.
  • Use strings instead of references (e.g. @OneToMany('Book', 'author')). The downside here is that you will loose the typechecking capabilities of the decorators.

Entity Discovery in TypeScript

Internally, MikroORM uses ts-morph to perform analysis of source files of entities to sniff types of all properties. This process can be slow if your project contains lots of files. To speed up the discovery process a bit, you can provide more accurate paths where your entity source files are:

const orm = await MikroORM.init({
entitiesDirs: ['./dist/entities'], // path to your JS entities (dist), relative to `baseDir`
entitiesDirsTs: ['./src/entities'], // path to your TS entities (source), relative to `baseDir`
// ...

You can also use different metadata provider or even write custom one:

  • ReflectMetadataProvider that uses reflect-metadata instead of ts-morph
  • JavaScriptMetadataProvider that allows you to manually provide the entity schema (mainly for Vanilla JS)
const orm = await MikroORM.init({
metadataProvider: ReflectMetadataProvider,
// ...

Setting up the Commandline Tool

MikroORM ships with a number of command line tools that are very helpful during development, like Schema Generator and Entity Generator. You can call this command from the NPM binary directory or use npx:

$ node node_modules/.bin/mikro-orm
$ npx mikro-orm

# or when installed globally
$ mikro-orm

For CLI to be able to access your database, you will need to create mikro-orm.config.js file that exports your ORM configuration. TypeScript is also supported, just enable useTsNode flag in your package.json file. There you can also set up array of possible paths to mikro-orm.config file, as well as use different file name:

Do not forget to install ts-node when enabling useTsNode flag.

"name": "your-app",
"dependencies": { ... },
"mikro-orm": {
"useTsNode": true,
"configPaths": [
export default {
entities: [Author, Book, BookTag],
dbName: 'my-db-name',
type: 'mongo', // one of `mongo` | `mysql` | `mariadb` | `postgresql` | `sqlite`

Once you have the CLI config properly set up, you can omit the MikroORM.init() options parameter and the CLI config will be automatically used.

You can also use different names for this file, simply rename it in the configPaths array your in package.json. You can also use MIKRO_ORM_CLI environment variable with the path to override configPaths value.

Now you should be able to start using the CLI. All available commands are listed in the CLI help:

Usage: mikro-orm <command> [options]

mikro-orm cache:clear Clear metadata cache
mikro-orm generate-entities Generate entities based on current database schema
mikro-orm schema:create Create database schema based on current metadata
mikro-orm schema:drop Drop database schema based on current metadata
mikro-orm schema:update Update database schema based on current metadata

-v, --version Show version number [boolean]
-h, --help Show help [boolean]

mikro-orm schema:update --run Runs schema synchronization

To verify your setup, you can use mikro-orm debug command.

When you have CLI config properly set up, you can omit the options parameter when calling MikroORM.init().

Request Context

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);

More info about RequestContext is described here.

Now you can start defining your entities (in one of the entitiesDirs folders).