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Separating Concerns using Embeddables

Support for embeddables was added in version 4.0

Embeddables are classes which are not entities themselves, but are embedded in entities and can also be queried. You'll mostly want to use them to reduce duplication or separating concerns. Value objects such as date range or address are the primary use case for this feature.

Embeddables needs to be discovered just like regular entities, don't forget to add them to the list of entities when initializing the ORM.

Embeddables can contain properties with basic @Property() mapping, nested @Embedded() properties or arrays of @Embedded() properties. From version 5.0 we can also use @ManyToOne() properties.

For the purposes of this tutorial, we will assume that you have a User class in your application, and you would like to store an address in the User class. We will model the Address class as an embeddable instead of simply adding the respective columns to the User class.

import { Embeddable, Embedded, Entity, PrimaryKey, Property } from '@mikro-orm/core';

@Embeddable()
export class Address {

  @Property()
  street!: string;

  @Property()
  postalCode!: string;

  @Property()
  city!: string;

  @Property()
  country!: string;

}

@Entity()
export class User {

  @PrimaryKey()
  id!: number;

  @Embedded(() => Address)
  address!: Address;

}

When using ReflectMetadataProvider, you might need to provide the class in decorator options: @Embedded(() => Address) or @Embedded({ entity: () => Address }).

In terms of your database schema, MikroORM will automatically inline all columns from the Address class into the table of the User class, just as if you had declared them directly there.

Initializing embeddables​

In case all fields in the embeddable are nullable, you might want to initialize the embeddable, to avoid getting a null value instead of the embedded object.

./entities/User.ts
@Embedded(() => Address)
address = new Address();

Column Prefixing​

By default, MikroORM names your columns by prefixing them, using the value object name.

Following the example above, your columns would be named as address_street, address_postal_code...

You can change this behaviour to meet your needs by changing the prefix attribute in the @Embedded() notation.

The following example shows you how to set your prefix to myPrefix_:

./entities/User.ts
@Entity()
export class User {

  @Embedded(() => Address, { prefix: 'myPrefix_' })
  address!: Address;

}

You can also decide more precisely how the column name is determined with an explicit prefix. With the example below:

  • absolute mode (default) sets the prefix at the beginning of the column, naming them addr_city, addr_street, ...
  • relative mode concatenates the prefix with its parent's prefix (if any), naming them contact_addr2_city, contact_addr2_street, ...
warning

The default value of prefixMode will change in v7 to relative.

./entities/User.ts
@Embeddable()
export class Contact {

  @Embedded({ entity: () => Address, prefix: 'addr_', prefixMode: 'absolute' })
  address!: Address;

  @Embedded({ entity: () => Address, prefix: 'addr2_', prefixMode: 'relative' })
  address2!: Address;

}

@Entity()
export class User {

  @Embedded(() => Contact)
  contact!: Contact;

}

The default behavior can be defined in the ORM configuration:

MikroORM.init({ embeddables: { prefixMode: 'absolute' } })

To have MikroORM drop the prefix and use the value object's property name directly, set prefix: false:

./entities/User.ts
@Embedded({ entity: () => Address, prefix: false })
address!: Address;

Storing embeddables as objects​

From MikroORM v4.2 we can also store the embeddable as an object instead of inlining its properties to the owing entity.

./entities/User.ts
@Embedded({ entity: () => Address, object: true })
address!: Address;

In SQL drivers, this will use a JSON column to store the value.

Only MySQL and PostgreSQL drivers support searching by JSON properties currently.

This part of documentation is highly inspired by doctrine tutorial as the behaviour here is pretty much the same.

Array of embeddables​

Embedded arrays are always stored as JSON. It is possible to use them inside nested embeddables.

./entities/User.ts
@Embedded(() => Address, { array: true })
addresses: Address[] = [];

Nested embeddables​

Starting with v4.4, we can also nest embeddables, both in inline mode and object mode:

import { Embeddable, Embedded, Entity, PrimaryKey, Property } from '@mikro-orm/core';

@Entity()
export class User {

  @PrimaryKey()
  id!: number;

  @Property()
  name!: string;

  @Embedded(() => Profile, { object: true, nullable: true })
  profile?: Profile;

}

@Embeddable()
export class Profile {

  @Property()
  username: string;

  @Embedded(() => Identity)
  identity: Identity;

  constructor(username: string, identity: Identity) {
    this.username = username;
    this.identity = identity;
  }

}

@Embeddable()
export class Identity {

  @Property()
  email: string;

  constructor(email: string) {
    this.email = email;
  }

}

Polymorphic embeddables​

Since v5, it is also possible to use polymorphic embeddables. This means we can define multiple classes for a single embedded property and the right one will be used based on the discriminator column, similar to how single table inheritance work.

import { Embeddable, Embedded, Entity, Enum, PrimaryKey, Property } from '@mikro-orm/core';

export enum AnimalType {
  CAT,
  DOG,
}

@Embeddable({ abstract: true, discriminatorColumn: 'type' })
export abstract class Animal {

  @Enum(() => AnimalType)
  type!: AnimalType;

  @Property()
  name!: string;

}

@Embeddable({ discriminatorValue: AnimalType.CAT })
export class Cat extends Animal {

  @Property({ nullable: true })
  canMeow?: boolean = true;

  constructor(name: string) {
    super();
    this.type = AnimalType.CAT;
    this.name = name;
  }

}

@Embeddable({ discriminatorValue: AnimalType.DOG })
export class Dog extends Animal {

  @Property({ nullable: true })
  canBark?: boolean = true;

  constructor(name: string) {
    super();
    this.type = AnimalType.DOG;
    this.name = name;
  }

}

@Entity()
export class Owner {

  @PrimaryKey()
  id!: number;

  @Property()
  name!: string;

  @Embedded(() => [Cat, Dog])
  pet!: Cat | Dog;

}