Skip to main content
Version: 4.3

Installation & Usage

First install the module via yarn or npm and do not forget to install the driver package 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:

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.

We can also provide paths where you store your entities via entities array. Internally it uses globby so we can use globbing patterns, including negative globs.

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

If you are experiencing problems with folder based discovery, try using mikro-orm debug CLI command to check what paths are actually being used.

Since v4, you can also use file globs, like ./dist/app/**/entities/*.entity.js.

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.

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 entity references in 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 lose the typechecking capabilities of the decorators.

Entity Discovery in TypeScript#

In v4 the default metadata provider is ReflectMetadataProvider. If you want to use ts-morph based discovery (that reads actual TS types via the compiler API), you need to install @mikro-orm/reflection.

import { TsMorphMetadataProvider } from '@mikro-orm/reflection';
const orm = await MikroORM.init({
metadataProvider: TsMorphMetadataProvider,
// ...

Read more about the differences in Metadata Providers section.

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

It is important that entities will point to the compiled JS files, and entitiesTs will point to the TS source files. You should not mix those.

For ts-morph discovery to work in production, we need to deploy .d.ts declaration files. Be sure to enable compilerOptions.declaration in your tsconfig.json.

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)

Using EntitySchema is another way to define your entities, which is better suited than using JavaScriptMetadataProvider.

const orm = await MikroORM.init({
// default in v4, so not needed to specify explicitly
metadataProvider: ReflectMetadataProvider,
// ...

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);

If the next handler needs to be awaited (like in Koa), use RequestContext.createAsync() instead.

app.use((ctx, next) => RequestContext.createAsync(orm.em, next));

More info about RequestContext is described here.

Setting up the Commandline Tool#

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

To work with the CLI, first install @mikro-orm/cli package locally. The version needs to be aligned with the @mikro-orm/core package.

# install the CLI package first!
$ yarn add @mikro-orm/cli
# manually
$ node node_modules/.bin/mikro-orm
# via npx
$ npx mikro-orm
# or via yarn
$ yarn 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.

MikroORM will always try to load the first available config file, based on the order in configPaths. This means that if you specify the first item as the TS config, but you do not have ts-node enabled and installed, it will fail to load it.

"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 cache:generate Generate metadata cache for production
mikro-orm generate-entities Generate entities based on current database
mikro-orm database:import <file> Imports the SQL file to the database
mikro-orm schema:create Create database schema based on current
mikro-orm schema:drop Drop database schema based on current
mikro-orm schema:update Update database schema based on current
mikro-orm migration:create Create new migration with current schema
mikro-orm migration:up Migrate up to the latest version
mikro-orm migration:down Migrate one step down
mikro-orm migration:list List all executed migrations
mikro-orm migration:pending List all pending migrations
mikro-orm debug Debug CLI configuration
-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().

Note: When importing a dump file you need multipleStatements: true in your configuration. Please check the configuration documentation for more information.

Now you can start defining your entities.

Last updated on by dependabot[bot]