Injectable EventSubscribers

Is your feature request related to a problem? Please describe. It would be awesome if EventSubscriber classes could support NestJs DI so we can inject dependencies in them.

Describe the solution you’d like The current @Subscriber() decorator cannot be used as it directly creates a new instance: https://github.com/mikro-orm/mikro-orm/blob/master/packages/core/src/decorators/Subscriber.ts#L8 And I didn’t find a clear solution to remove the attached subscriber before MikroORM init.

I think the easiest way is to have a new decorator (could be @InjectableSubscriber(), that also wraps Nestjs @Injectable() ?) that simply adds a metadata to the class prototype flagging it as a Subscriber, then the class has to be registered as a provider inside a Module (like any nestjs providers).

Then follow a common Nestjs pattern, “disover” this providers via the custom metadata and do stuff with them, in our case register them as subscriber (em.getEventManager().registerSubscriber()).

Additional context As a reference, @nestjs/bull does the same for registering queue processors https://github.com/nestjs/bull/blob/master/lib/bull.explorer.ts . It is a bit more complicated since it has to check every class methods instead of just the class, what we need are just the first lines of explore():

const providers: InstanceWrapper[] = this.discoveryService
      .getProviders()
      .filter((wrapper: InstanceWrapper) =>
        this.metadataAccessor.isQueueComponent(wrapper.metatype),
      );

Replace isQueueComponent with something like isSubscriberComponent and voilà, we have an array of all provided subscribers.

[B4nan]

This is already possible:

@Injectable()
export class MySubscriber {

  constructor(em: EntityManager) {
    em.getEventManager().registerSubscriber(this);
  }

}

[EricMcRay]

This is already possible:

@Injectable()
export class MySubscriber {

  constructor(em: EntityManager) {
    em.getEventManager().registerSubscriber(this);
  }

}

Can you also put this to documents? I was looking for this solution for hours.