Projection with custom properties

When auto-mapping isn't sufficient, you can explicitly map properties from events to your read model. This gives you full control over how data is transformed and mapped.

Explicit property mapping

Instead of using AutoMap(), use Set() methods to explicitly define property mappings:

using Cratis.Chronicle.Projections;

public class AccountProjection : IProjectionFor<Account>
{
    public void Define(IProjectionBuilderFor<Account> builder) => builder
        .From<AccountOpened>(_ => _
            .Set(m => m.AccountNumber).To(e => e.Number)
            .Set(m => m.CustomerName).To(e => e.Owner.Name)
            .Set(m => m.Balance).To(42.0m)
            .Set(m => m.IsActive).To(true)
            .Set(m => m.OpenedAt).To(e => e.Timestamp))
        .From<MoneyDeposited>(_ => _
            .Set(m => m.Balance).To(e => e.Amount)
            .Set(m => m.LastTransaction).To(e => e.Timestamp));
}

Combining AutoMap with explicit mapping

You can use AutoMap() at the top level to automatically map matching properties, then add explicit mappings for specific transformations:

public class AccountProjection : IProjectionFor<Account>
{
    public void Define(IProjectionBuilderFor<Account> builder) => builder
        .AutoMap()  // Automatically maps matching properties
        .From<AccountOpened>(_ => _
            .Set(m => m.CustomerName).To(e => e.Owner.Name)  // Custom mapping for nested property
            .Set(m => m.IsActive).To(true))                  // Custom mapping for constant
        .From<MoneyDeposited>();  // Uses AutoMap for all properties
}

AutoMap() works recursively, automatically mapping:

Properties with matching names and compatible types
Nested objects and their properties
Collections and arrays

You can also use AutoMap() explicitly for each event type instead of at the projection level:

.From<AccountOpened>(_ => _.AutoMap())
.From<MoneyDeposited>(_ => _.AutoMap())

Read model definition

The read model can have different property names and types than the events:

public record Account(
    string AccountNumber,
    string CustomerName,
    decimal Balance,
    bool IsActive,
    DateTimeOffset OpenedAt,
    DateTimeOffset? LastTransaction);

Event definitions

Events can have different structures than the read model:

[EventType]
public record AccountOpened(
    string Number,
    Customer Owner,
    DateTimeOffset Timestamp);

[EventType]
public record MoneyDeposited(
    decimal Amount,
    DateTimeOffset Timestamp);

public record Customer(string Name, string Email);

Property mapping options

You can map properties in several ways:

From event property: .Set(m => m.CustomerName).To(e => e.Owner.Name)
From constant value: .Set(m => m.IsActive).To(true)
From computed value: .Set(m => m.DisplayName).To(e => $"{e.FirstName} {e.LastName}")
From event source ID: .Set(m => m.Id).ToEventSourceId()

Multiple events

A single projection can handle multiple event types, each with its own property mappings. Properties are updated incrementally as events are processed.

In the example above:

AccountOpened sets initial values for all properties
MoneyDeposited only updates Balance and LastTransaction
Other properties retain their previous values

This approach gives you precise control over how your read models are built from events.