Projections

Projections are shared Chronicle read-model behavior. Use the shared docs for projection styles, model-bound projections, declarative projections, and client-tabbed examples.

• Projections
• Choosing a read model style
• Model-bound projections
• Declarative projections
• Kotlin and Java client setup

Kotlin model-bound attributes

Beyond @FromEvent and @SetFrom, the Kotlin client has annotations for structural shapes (joins, children, nested objects), arithmetic (counters, running totals), catch-all mappings, and rewind behavior. Full parameter tables are in the annotation reference.

Attribute	Use it for
@Join	Pulling in a property from another event type by id.
@ChildrenFrom	A collection built from child instances of an event type.
@Nested	A nullable sub-object built from its own @FromEvent.
@ClearWith	Which event clears (nulls out) a @Nested property.
@Count	An occurrence counter for a specific event type.
@Increment / @Decrement	Bumping a numeric property by one.
@AddFrom / @SubtractFrom	Adding/subtracting an event value in.
@FromAll / @FromEvery	A property populated from every event type.
@NotRewindable	Marking a projection as forward-only.
@RemovedWith	Which event removes an instance or a child.
@RemovedWithJoin	Like @RemovedWith, but resolving the id via a join.
@NoAutoMap	Disabling AutoMap for a type or a single property.

@NotRewindable is worth considering for projections fed by events that can’t reliably be redelivered — e.g. from an event store subscription.

import io.cratis.chronicle.projections.Increment
import io.cratis.chronicle.projections.Join
import io.cratis.chronicle.readModels.ReadModel

@ReadModel
data class OrderOverview(
    val id: String = "",
    @Increment(OrderShipped::class) val shipmentCount: Int = 0,
    @Join(
        eventType = CustomerRegistered::class,
        on = "customerId",
        eventPropertyName = "email"
    )
    val customerEmail: String = ""
)

Richer declarative projections

IProjectionBuilderFor<T> also supports .join(), .fromEvery()/.fromAll(), .removedWith()/.removedWithJoin(), .children(), .nested(), and .notRewindable() — the fluent equivalents of the attributes above, for projections defined with a separate IProjectionFor<T> class instead of model-bound annotations:

import io.cratis.chronicle.projections.IProjectionBuilderFor
import io.cratis.chronicle.projections.IProjectionFor

class OrderOverviewProjection : IProjectionFor<OrderOverview> {
    override fun define(builder: IProjectionBuilderFor<OrderOverview>) {
        builder
            .from(OrderPlaced::class)
            .join(CustomerRegistered::class) { join ->
                join.on(OrderOverview::customerEmail)
                    .set(OrderOverview::customerEmail)
                    .toProperty("email")
            }
            .notRewindable()
    }
}