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Cratis Cratis Cratis

CHR0041: Event filter attribute on a projection has no effect

Rule Description

Section titled “Rule Description”

An event-metadata filter attribute — [EventStreamType], [EventSourceType], or [FilterEventsByTag] — is applied to a projection, where it does nothing. A projection observes every event of the types its definition declares; there is no field for a metadata filter anywhere in a projection’s definition, so no client can express one and no kernel could honor it.

Filtering observed events on metadata is what reactors and reducers do — see appended event metadata filters. To narrow a projection, narrow the event types it declares, or pair it with a reactor or reducer that owns the filtered subset.

Both projection spellings are covered: a model-bound [ReadModel] carrying projection attributes, and a fluent IProjectionFor<T>.

Severity

Section titled “Severity”

Warning

Example

Section titled “Example”
using Cratis.Chronicle;
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Keys;
using Cratis.Chronicle.Projections.ModelBound;


[EventType]
public record Chr0041TemperatureRead(double Degrees);


// Warning CHR0041: '[FilterEventsByTag]' on projection 'Chr0041LatestTemperature' has no
// effect - a projection observes every event of the types it declares and cannot filter
// on event metadata. Readings from every sensor land in this read model, not just sensor-a's.
[FilterEventsByTag("sensor-a")]
[FromEvent<Chr0041TemperatureRead>]
public record Chr0041LatestTemperature(
    [Key] Guid Id,
    double Degrees);

Why This Rule Exists

Section titled “Why This Rule Exists”

The filter attributes’ own documentation used to say a projection could filter, and that is what an IDE tooltip shows at the moment of authoring. A developer who trusts it designs a filtered projection that silently observes everything — no build error, no startup failure, no runtime signal, just a read model carrying values from streams it was never meant to see. The natural way to debug that — re-reading the attribute to confirm it is present and spelled right — confirms the wrong conclusion.

Correcting the documentation helps the reader who reads it; this rule helps the one who does not.

Section titled “Related Rules”