_06-read-models.md

Read Models - The Read Pipeline

TODO: (Not in the current release) To generate a Read Model you can use a read model generator. It works similarly than the entities generator:

boost new:read-model <name of the read model class> --fields fieldName:fieldType --projects EntityOne EntityTwo

Using the generator will generate a class with the following structure in src/read-models/<name-of-the-read-model>.ts. For instance:

boost new:read-model CartReadModel --fields id:UUID cartItems:"Array<CartItem>" paid:boolean --projects Cart

It will generate a class with the following structure:

@ReadModel
export class CartReadModel {
  public constructor(
    readonly id: UUID,
    readonly cartItems: Array<CartItem>,
    public paid: boolean
  ) {}

  @Projection(Cart, 'id')
  public static updateWithCart(cart: Cart, oldCartReadModel?: CartReadModel): CartReadModel {
    return new CartReadModel(cart.id, cart.cartItems, cart.paid)
  }
}

Read Models are cached data optimized for read operations and they're updated reactively when Entities are updated by new events. They also define the Read API, the available REST endpoints and their structure.

Read Models are classes decorated with the @ReadModel decorator that have one or more projection methods. A Projection is a method decorated with the @Projection decorator that, given a new entity value and (optionally) a previous read model state, generate a new read model value.

Read models can be projected from multiple entities as soon as they share some common key called joinKey.

Read Model classes can also be created by hand and there are no restrictions regarding the place you put the files. The structure of the data is totally open and can be as complex as you can manage in your projection functions.

Defining a read models enables a new REST Read endpoint that you can use to query or poll the read model records see the API documentation.