Inheritance of AggregateRoots and Emit issue

[Martin Potočnik]

Hello,

First of all, Cirqus is great and thank you for dev it! We use it a lot and recently came across an issue with inheritance. Is there any reason why aggregates should not emit events from aggregates extending them?

Example:

Base aggregate & event:
public class Device : AggregateRoot {}
public class DeviceChanged : DomainEvent

Two classes extending the base aggregate:
public class SpecialDeviceA : Device {}
public class SpecialDeviceB : Device {}

Although SpecialDeviceA is Device, if I create a SpecialDeviceA and call some method implemented by Device (base) emitting DeviceChanged event I get:
System.InvalidOperationException: Attempted to emit event “DeviceChanged” which is owned by “Device” from aggregate root of type “SpecialDeviceA”.

For now I fixed it with checking if Agg type is assignable from X (and only the exact type of X) in the d60.Cirqus.Aggregates.AggregateRoot.cs, line 81:

From: if (typeof(TAggregateRoot) != GetType())
To: if(!typeof(TAggregateRoot).IsAssignableFrom(GetType()))

Aggregate tests still pass :)

Is this ok? As we (and probably others too) use inheritance with Aggregates (nature of our domain), maybe Cirqus could enable this by default?

Kind regards,
Martin

[Mogens Heller Grabe]

Oh, that is definitely ok 👍 if you submit a PR, I'd be happy to merge it :)

[Asger Hallas]

Should the Owner meta data of the event be set to the emitting type instead of the owning type then? I believe it's used when instantiating roots and that needs to be the concrete type.

[Mogens Heller Grabe]

Yes, it should be the concrete type of the aggregate root, no matter if it's inherited off of some kind of base class.

[Asger Hallas]

I'm thinking of this line in RealUnitOfWork.AddEmittedEvent():

e.Meta[DomainEvent.MetadataKeys.Owner] = _typeNameMapper.GetName(typeof (TAggregateRoot));

It takes the type of the owner defined by the event - in this case the base type Device. That means that the DefaultAggregateRootRepository.Get() will try to create an instance of the base type here:

var aggregateRootTypeName = e.Meta[DomainEvent.MetadataKeys.Owner];
var aggregateRootType = _domainTypeNameMapper.GetType(aggregateRootTypeName);
aggregateRoot = CreateNewAggregateRootInstance(aggregateRootType, aggregateRootId, unitOfWork);

So I AddEmittedEvent ought to by changed, I believe, to make it work.

[Asger Hallas]

As an aside, I guess Emitter would be a better name for the metadata key. But I guess that'll break some running systems :)

[Matej Hertis]

I implemented this with suggested emitter metadata and it seems to work.

In AggregateRoot

            e.Meta[DomainEvent.MetadataKeys.Emitter] = GetType().AssemblyQualifiedName;

In RealUnitOfWork

            var emitterType = Type.GetType(e.Meta[DomainEvent.MetadataKeys.Emitter]);
            e.Meta[DomainEvent.MetadataKeys.Owner] = _typeNameMapper.GetName(emitterType);

Now the problem is that I cannot get type of concrete aggregate root in CirqusTestsHarness.

     void Emit<T>(string id, DomainEvent<T> @event) where T : AggregateRoot
        {
            @event.Meta[DomainEvent.MetadataKeys.AggregateRootId] = id;
            ..
        }

Any idea?

[Matej Hertis]

Eventually, I came up with some kind of solution. Please check out latest commits.

[Asger Hallas]

I see the problem in the test harness, I think your solution is the right one. But I have a few comments on the UnitOfWork... I'll leave them in the commits :)

[Asger Hallas]

I said a few, but in reality just one.

It's a tough one with the TestContext and the harness. I believe that emitted events should not have an owning type at all, but that might be a discussion for another day :)

[Matej Hertis]

Take 2, this time without Emitter :) Still some room for improvments in test harness.