cratis-readmodel

Creating a Cratis Read Model

A read model is derived state built from events. The path is:

[EventType] records  →  [ReadModel] record + static query methods  →  projection or reducer  →  TypeScript proxy  →  React

---

Step 1 — Define your events

Events are the source of truth. Define each as a record decorated with [EventType]. Name them in past tense.

// Accounts/AccountSummary/AccountSummary.cs — events live in the slice file they belong to
using Cratis.Chronicle.Events;

/// <summary>Emitted when a debit account is opened.</summary>
[EventType]
public record DebitAccountOpened(AccountName Name, OwnerId OwnerId);

/// <summary>Emitted when a debit account is closed.</summary>
[EventType]
public record DebitAccountClosed;

/// <summary>Emitted when funds are deposited.</summary>
[EventType]
public record FundsDeposited(Money Amount);

/// <summary>Emitted when funds are withdrawn.</summary>
[EventType]
public record FundsWithdrawn(Money Amount);

Good event design:

• One clear purpose per event — do not mix concerns.
• Avoid nullable properties — Chronicle's analyzer warns on them; model an optional fact as a separate event.
• Properties are concept-typed facts (never raw Guid/string), and never carry the event-source id.

---

Step 2 — Define the read model record

Decorate the record with [ReadModel] and add static query methods directly on it. The proxy generator turns each static method into a TypeScript query class.

// Domain/ReadModels/AccountSummary.cs
using Cratis.Arc.Queries.ModelBound;
using MongoDB.Driver;

[ReadModel]
public record AccountSummary(AccountId Id, string Name, OwnerId OwnerId, decimal Balance, bool IsClosed)
{
    // Snapshot query — returns current data once
    public static async Task<IEnumerable<AccountSummary>> AllAccounts(
        IMongoCollection<AccountSummary> collection)
        => await collection.Find(Builders<AccountSummary>.Filter.Empty).ToListAsync();

    public static async Task<AccountSummary?> GetAccount(
        AccountId id,
        IMongoCollection<AccountSummary> collection)
        => await collection.Find(a => a.Id == id).FirstOrDefaultAsync();

    // Observable query — pushes updates in real time
    public static ISubject<IEnumerable<AccountSummary>> ObserveAllAccounts(
        IMongoCollection<AccountSummary> collection)
        => collection.Observe();
}