cratis-chronicle-projection

Chronicle projections

A projection consumes events and produces read-model state. It is pure: no side effects, and it never reads another read model. Prefer model-bound attributes on the read model; fall back to the fluent builder only when the attributes cannot express the shape; reach for a reducer only after both fail.

Verified product sources

This skill is verified against this exact source:

Package	Version	Purpose
Cratis.Chronicle	16.45.2	Cratis.Chronicle.Projections, Cratis.Chronicle.Projections.ModelBound, Cratis.Chronicle.Keys, Cratis.Chronicle.EventSequences

Reverify product sources before claiming support for another version.

[ReadModel] and [Path] are Arc types (Cratis.Arc.Queries.ModelBound), not Chronicle types. A Chronicle read model that exists only as a projection target needs no [ReadModel] at all — that attribute is what gives it an Arc query surface.

Model-bound projections — the default

Put the projection on the read model with attributes. No separate class.

using Cratis.Chronicle.Keys;
using Cratis.Chronicle.Projections.ModelBound;

[FromEvent<<EventName>>]
public record <ReadModelName>(
    [property: Key] <IdType> Id,
    <PropertyType> <PropertyName>);

Matching property names map automatically. AutoMap is on by default — both for model-bound types and for the fluent builder — so never call .AutoMap() except inside a scope where you disabled it.

[Key] is Cratis.Chronicle.Keys.KeyAttribute. It has no arguments and targets a property or a record constructor parameter, never the class.

Read references/model-bound-attributes.md for the complete attribute list with exact arities, parameter names, and valid targets. Two things there are worth knowing before you write anything:

• [Nested], [FromAll], and [FromEvery] are non-generic.
• ConstantKey on [Count<T>], [Increment<T>], [Decrement<T>], and [FromEvent<T>] is an object-initializer property, never a constructor argument: [Count<TEvent>(ConstantKey = "<key>")].