write-specs-readmodels

name	write-specs-readmodels
description	Use this skill when asked to write tests or specs for read model projections, reducers, or model-bound projections using ReadModelScenario in a Cratis-based project. Produces infrastructure-free in-process specs using ReadModelScenario<TReadModel>.

Writing Read Model Specs with ReadModelScenario

Use ReadModelScenario<TReadModel> to test read model projections and reducers entirely in-process — no Chronicle server, database, or network required.

When to use this skill

• Testing that a reducer correctly builds state from a sequence of events
• Testing that a fluent IProjectionFor<T> projection maps event properties to read model properties
• Testing that a model-bound projection ([FromEvent<T>], [Key]) maps correctly
• Testing boundary conditions: first event, multiple events, different event sources

For testing event appending, constraints, or concurrency, use EventScenario instead.

---

Package

dotnet add package Cratis.Chronicle.Testing

---

Basic structure

// In your spec file (follows BDD Establish/Because/should_ pattern)
public class when_<event_sequence_description> : Specification
{
    ReadModelScenario<TReadModel> _scenario;

    async Task Establish()
    {
        _scenario = new ReadModelScenario<TReadModel>();
        await _scenario.Given
            .ForEventSource(<eventSourceId>)
            .Events(new <EventType>(<args>));
    }

    [Fact] void should_<expected_state>() => _scenario.Instance!.<Property>.ShouldEqual(<expectedValue>);
}

---

Step 1 — Create the scenario

Always create a new ReadModelScenario<TReadModel> per test. Optionally pass an initial state:

// Default (empty) initial state
var scenario = new ReadModelScenario<MyReadModel>();

// With an initial state baseline
var scenario = new ReadModelScenario<MyReadModel>(new MyReadModel { Count = 10 });

---

Step 2 — Seed events via the fluent Given builder

await scenario.Given
    .ForEventSource(myId)
    .Events(new SomeEvent("value"), new SomeOtherEvent(42));

Chain multiple ForEventSource calls to seed different event sources:

await scenario.Given
    .ForEventSource(orderId)
    .Events(new OrderCreated("order-1"), new ItemAdded(9.99m));

await scenario.Given
    .ForEventSource(anotherOrderId)
    .Events(new OrderCreated("order-2"));

Rules:

• Call Given before asserting — events are processed in the order they are supplied.
• Do not put the act-under-test inside Given; the Given call is the act for a read model spec.

---

Step 3 — Assert on Instance

_scenario.Instance!.Total.ShouldEqual(14.49m);
_scenario.Instance!.Name.ShouldEqual("Widget");
_scenario.Instance.ShouldNotBeNull();

---

Auto-detection order

ReadModelScenario<TReadModel> searches loaded assemblies for a handler in this order:

1. Reducer — a class implementing IReducerFor<TReadModel>
2. Fluent projection — a class implementing IProjectionFor<TReadModel>
3. Model-bound projection — TReadModel carries [FromEvent<T>] or [Key] attributes

If none is found, NoReadModelHandlerFound is thrown.

---

Example: reducer

public class when_items_are_added_to_an_order : Specification
{
    ReadModelScenario<OrderSummary> _scenario;
    static readonly string OrderId = "order-1";

    async Task Establish()
    {
        _scenario = new ReadModelScenario<OrderSummary>();
        await _scenario.Given
            .ForEventSource(OrderId)
            .Events(
                new OrderCreated(OrderId),
                new ItemAdded(9.99m),
                new ItemAdded(4.50m));
    }

    [Fact] void should_sum_the_item_prices() => _scenario.Instance!.Total.ShouldEqual(14.49m);
    [Fact] void should_have_the_correct_order_id() => _scenario.Instance!.OrderId.ShouldEqual(OrderId);
}

---

Example: fluent projection

public class when_a_product_is_created_and_stock_is_adjusted : Specification
{
    ReadModelScenario<ProductView> _scenario;

    async Task Establish()
    {
        _scenario = new ReadModelScenario<ProductView>();
        await _scenario.Given
            .ForEventSource("product-1")
            .Events(
                new ProductCreated("Widget"),
                new StockAdjusted(100));
    }

    [Fact] void should_set_the_name() => _scenario.Instance!.Name.ShouldEqual("Widget");
    [Fact] void should_set_the_stock() => _scenario.Instance!.Stock.ShouldEqual(100);
}

---

Example: model-bound projection

public class when_a_shipment_is_dispatched_and_delivered : Specification
{
    ReadModelScenario<DeliveryStatus> _scenario;
    static readonly DateTimeOffset DeliveredAt = DateTimeOffset.UtcNow;

    async Task Establish()
    {
        _scenario = new ReadModelScenario<DeliveryStatus>();
        await _scenario.Given
            .ForEventSource("shipment-1")
            .Events(
                new ShipmentDispatched("FedEx"),
                new ShipmentDelivered(DeliveredAt));
    }

    [Fact] void should_set_the_carrier() => _scenario.Instance!.Carrier.ShouldEqual("FedEx");
    [Fact] void should_record_the_delivery_time() => _scenario.Instance!.DeliveredAt.ShouldEqual(DeliveredAt);
}

---

Example: initial state (baseline)

public class when_an_item_is_added_to_a_cart_with_existing_items : Specification
{
    ReadModelScenario<CartSummary> _scenario;

    async Task Establish()
    {
        var initial = new CartSummary { ItemCount = 3 };
        _scenario = new ReadModelScenario<CartSummary>(initial);
        await _scenario.Given
            .ForEventSource("cart-1")
            .Events(new ItemAddedToCart("item-4"));
    }

    [Fact] void should_increment_the_item_count() => _scenario.Instance!.ItemCount.ShouldEqual(4);
}

---

Checklist

• New ReadModelScenario<T> instance per test (never shared state)
• Given.ForEventSource(id).Events(...) called before asserting
• Each test asserts a single outcome (one should_ per behavior)
• Instance is accessed with ! only after Given has been called
• Run dotnet build and dotnet test — fix all failures before completing