Lifecycle Hooks and EventSubscriber

There are two ways to hook to the lifecycle of an entity:

• Lifecycle hooks are methods defined on the entity prototype.
• EventSubscribers are classes that can be used to hook to multiple entities or when you do not want to have the method present on the entity prototype.

Hooks are internally executed the same way as subscribers.

Hooks are executed before subscribers.

Hooks

You can use lifecycle hooks to run some code when entity gets persisted. You can mark any of entity methods with them, you can also mark multiple methods with same hook.

All hooks support async methods with one exception - @OnInit.

• @OnInit is fired when new instance of entity is created, either manually em.create(), or automatically when new entities are loaded from database

• @BeforeCreate() and @BeforeUpdate() is fired right before we persist the entity in database

• @AfterCreate() and @AfterUpdate() is fired right after the entity is updated in database and merged to identity map. Since this event entity will have reference to EntityManager and will be enabled to call wrap(entity).init() method (including all entity references and collections).

• @BeforeDelete() is fired right before we delete the record from database. It is fired only when removing entity or entity reference, not when deleting records by query.

• @AfterDelete() is fired right after the record gets deleted from database and it is unset from the identity map.

@OnInit is not fired when you create the entity manually via its constructor (new MyEntity())

@OnInit can be sometimes fired twice, once when the entity reference is created, and once after its populated. To distinguish between those we can use wrap(this).isInitialized().

Limitations of lifecycle hooks

Hooks are executed inside the commit action of unit of work, after all change sets are computed. This means that it is not possible to create new entities as usual from inside the hook. Calling em.flush() from hooks will result in validation error. Calling em.persist() can result in undefined behaviour like locking errors.

The internal instance of EntityManager accessible under wrap(this, true).__em is not meant for public usage.

EventSubscriber

Use EventSubscriber to hook to multiple entities or if you do not want to pollute the entity prototype. All methods are optional, if you omit the getSubscribedEntities() method, it means you are subscribing to all entities.

You can either register the subscribers manually in the ORM configuration (via subscribers array where you put the instance):

MikroORM.init({
  subscribers: [new AuthorSubscriber()],
});

Or use @Subscriber() decorator - keep in mind that you need to make sure the file gets loaded in order to make this decorator registration work (e.g. you import that file explicitly somewhere).

import { EntityName, EventArgs, EventSubscriber, Subscriber } from '@mikro-orm/core';

@Subscriber()
export class AuthorSubscriber implements EventSubscriber<Author> {

  getSubscribedEntities(): EntityName<Author>[] {
    return [Author];
  }

  async afterCreate(args: EventArgs<Author>): Promise<void> {
    // ...
  }

  async afterUpdate(args: EventArgs<Author>): Promise<void> {
    // ...
  }

}

Another example, where we register to all the events and all entities:

import { EventArgs, EventSubscriber, Subscriber } from '@mikro-orm/core';

@Subscriber()
export class EverythingSubscriber implements EventSubscriber {

  async beforeCreate<T>(args: EventArgs<T>): Promise<void> { ... }
  async afterCreate<T>(args: EventArgs<T>): Promise<void> { ... }
  async beforeUpdate<T>(args: EventArgs<T>): Promise<void> { ... }
  async afterUpdate<T>(args: EventArgs<T>): Promise<void> { ... }
  async beforeDelete<T>(args: EventArgs<T>): Promise<void> { ... }
  async afterDelete<T>(args: EventArgs<T>): Promise<void> { ... }
  async beforeFlush<T>(args: EventArgs<T>): Promise<void> { ... }
  async onFlush<T>(args: EventArgs<T>): Promise<void> { ... }
  async afterFlush<T>(args: EventArgs<T>): Promise<void> { ... }
  onInit<T>(args: EventArgs<T>): void { ... }

}

EventArgs

As a parameter to the hook method we get EventArgs instance. It will always contain reference to the current EntityManager and the particular entity. Events fired from UnitOfWork during flush operation also contain the ChangeSet object.

interface EventArgs<T> {
  entity: T;
  em: EntityManager;
  changeSet?: ChangeSet<T>;
}

interface ChangeSet<T> {
  name: string;                   // entity name
  collection: string;             // db table name
  type: ChangeSetType;            // type of operation
  entity: T;                      // up to date entity instance
  payload: EntityData<T>;         // changes that will be used to build the update query
  persisted: boolean;             // whether the changeset was already persisted/executed
  originalEntity?: EntityData<T>; // snapshot of the entity when it was loaded from db
}

enum ChangeSetType {
  CREATE = 'create',
  UPDATE = 'update',
  DELETE = 'delete',
}

Flush events

There is a special kind of events executed during the commit phase (flush operation). They are executed before, during and after the flush, and they are not bound to any entity in particular.

• beforeFlush is executed before change sets are computed, this is the only event where it is safe to persist new entities.
• onFlush is executed after the change sets are computed.
• afterFlush is executed as the last step just before the flush call resolves. it will be executed even if there are no changes to be flushed.

Flush event args will not contain any entity instance, as they are entity agnostic. They do contain additional reference to the UnitOfWork instance.

interface FlushEventArgs extends Omit<EventArgs<unknown>, 'entity'> {
  uow?: UnitOfWork;
}

Flush events are entity agnostic, specifying getSubscribedEntities() method will not have any effect for those. They are fired only once per the flush operation.

Getting the changes from UnitOfWork

You can observe all the changes that are part of given UnitOfWork via those methods:

UnitOfWork.getChangeSets(): ChangeSet<AnyEntity>[];
UnitOfWork.getOriginalEntityData(): Map<string, EntityData<AnyEntity>>;
UnitOfWork.getPersistStack(): Set<AnyEntity>;
UnitOfWork.getRemoveStack(): Set<AnyEntity>;
UnitOfWork.getCollectionUpdates(): Collection<AnyEntity>[];
UnitOfWork.getExtraUpdates(): Set<[AnyEntity, string, (AnyEntity | Reference<AnyEntity>)]>;

Using onFlush event

In following example we have 2 entities: FooBar and FooBaz, connected via M:1 relation. Our subscriber will automatically create new FooBaz entity and connect it to the FooBar when we detect it in the change sets.

We first use uow.getChangeSets() method to look up the change set of entity we are interested in. After we create the FooBaz instance and link it with FooBar, we need to do two things:

1. Call uow.computeChangeSet(baz) to compute the change set of newly created FooBaz entity
2. Call uow.recomputeSingleChangeSet(cs.entity) to recalculate the existing change set of the FooBar entity.

@Subscriber()
export class FooBarSubscriber implements EventSubscriber {

  async onFlush(args: FlushEventArgs): Promise<void> {
    const changeSets = args.uow.getChangeSets();
    const cs = changeSets.find(cs => cs.type === ChangeSetType.CREATE && cs.entity instanceof FooBar);

    if (cs) {
      const baz = new FooBaz();
      baz.name = 'dynamic';
      cs.entity.baz = baz;
      args.uow.computeChangeSet(baz);
      args.uow.recomputeSingleChangeSet(cs.entity);
    }
  }

}

const bar = new FooBar();
bar.name = 'bar';
await em.persistAndFlush(bar);