Deleting a projection

[AlexZeitler]

Whats the common way to delete a projection based on a particular Event?

[jeremydmiller]

@AlexZeitler Are you using one of the 4.0 alphas or back on 3.*?

[jeremydmiller]

Okay, cool.

So if your projection is any kind of aggregate document and you're using the AggregateProjection<T> or ViewProjection<T, TId>, you can either use the DeleteEvent<TEventType>() syntax to mean "always delete the aggregate for this stream or group of events" as shown in this example:

    public class TripAggregationWithoutCustomName: AggregateProjection<Trip>
    {
        public TripAggregationWithoutCustomName()
        {
            DeleteEvent<TripAborted>();

            // Now let's change the lifecycle to inline
            Lifecycle = ProjectionLifecycle.Inline;
        }

        public void Apply(Arrival e, Trip trip) => trip.State = e.State;
        public void Apply(Travel e, Trip trip) => trip.Traveled += e.TotalDistance();
        public void Apply(TripEnded e, Trip trip)
        {
            trip.Active = false;
            trip.EndedOn = e.Day;
        }

        public Trip Create(TripStarted started)
        {
            return new Trip {StartedOn = started.Day, Active = true};
        }
    }

Or if it's conditional based on some state of the aggregate or the event, you can use the ShouldDelete() syntax:

        public bool ShouldDelete(MyAggregate aggregate, EEvent @event)
        {
            // Use some kind of conditional logic to decide if it gets deleted or not. Can use any mix of the aggregate, the raw event data, IEvent, IEvent<T>, and IQuerySession as arguments here.
        }

[AlexZeitler]

Thanks for the explanation.

For now I've chosen the ShouldDelete option as I didn't want to use inheritance and it just worked like this in F#

type CustomerReadModel() = 
    member val Id = Guid.Empty with get,set
    member val Version = 0L with get,set
    member val CompanyName = String.Empty with get,set
    member this.Apply(event: Events.IEvent<CustomerRegisteredEvent>) =
      this.Version <- event.Version
      let data: CustomerRegisteredEvent = event.Data |> unbox
      this.CompanyName <- data.CompanyName
    member this.ShouldDelete(event: Events.IEvent<CustomerDeletedEvent>) =
      true

[jeremydmiller]

Did that actually work? Was this through the SelfAggregate usage? I'm pleasantly surprised that worked. You could take in ICustomerDeletedEvent if you don't care about the event metadata to simplify things.

[AlexZeitler]

Yes, it did work, tried it several times.

That's how I'm registering the Projections and Events:

options.Events.Projections.SelfAggregate<Domain.CustomerReadModel>(ProjectionLifecycle.Inline) |> ignore
options.Events.AddEventType(typeof<Domain.CustomerRegisteredEvent>)
options.Events.AddEventType(typeof<Domain.CustomerDeletedEvent>)

[jeremydmiller]

Well, cool. I'll take the win on that one.