Projection

Projection is an Observer, that listens to events and updates an associated View.

Projection View Restoring

By default, an InMemoryView is used. That means that upon application start, Projection queries all known events from the EventStore and projects them to the view. Once this process is complete, the view's ready property gets switched from false to true.

Projection Event Handlers

All projection event types must be listed in the static handles getter and event type must have a handler defined:

const { AbstractProjection } = require('node-cqrs');

class MyProjection extends AbstractProjection {
  static get handles() {
    return [
      'userSignedUp',
      'userPasswordChanged'
    ];
  }

  async userSignedUp({ aggregateId, payload }) {
    const { profile, passwordHash } = payload;

    await this.view.create(aggregateId, {
      profile,
      passwordHash
    });
  }

  async userPasswordChanged({ aggregateId, payload }) {
    const { passwordHash } = payload;
    await this.view.update(aggregateId, view => {
      view.passwordHash = passwordHash;
    });
  }
}

Accessing Projection View

Associated view is exposed on a projection instance as view property.

By default, AbstractProjection instances get created with an instance of InMemoryView associated.