Is it possible to use SingleStreamProjection<T> with snapshots?

[federico-paolillo]

I want to enable snapshots on a document for which I have already a class extending SingleStreamProjection<T>. This class is named SensorProjection and is defined defined as follows:

public sealed class SensorProjection : SingleStreamProjection<Sensor>

I wanted to reuse SensorProjection so I tried to register it as a snapshot as follows:

Projections.Snapshot<SensorProjection>(SnapshotLifecycle.Inline);

This fails. Marten complains that SensorProjection does not have suitable Apply() methods because Marten expects the Apply() method to receive the same T configured with Projections.Snapshot<T>. It seems that Marten instantiates a SingleStreamProjection<T> when configuring snapshots.

Is there a way to use SingleStreamProjection<T> with snapshots or only document that directly mutates themselves are currently supported ?

[mysticmind]

You have to understand the concepts of usage clearly, outlining it below:

• Projections are read models created by projecting/shaping events optimized for querying data.
• Snapshots are used on the write model side to optimally regenerate an aggregate state. Aggregate encapsulate your business logic and invariants (specific rules or conditions that must always hold true within a given system or domain) of your domain. So when you are writing/updating on the write model side, you will always validate against the current state of an aggregate to ensure data consistency at the aggregate level.

Conceptually both Projections and Snapshots are very different in terms of their usage. That said, Marten may internally use the constructs to provide these functionality but as an end user you can't interchangeably use these as you wish.

[jeremydmiller]

@federico-paolillo If you're putting the Apply() type methods directly on Sensor, just use:

opts.Projections.SelfAggregate<Sensor>(lifecycle)