Event Projections

Event projections process individual events to create, modify, or delete documents. Unlike aggregate projections, they don't maintain per-stream state.

Defining an Event Projection

public class AuditLogProjection : EventProjection
{
    public void Project(IEvent<OrderCreated> @event, IDocumentOperations ops)
    {
        ops.Store(new AuditEntry
        {
            Id = Guid.NewGuid(),
            Action = "OrderCreated",
            StreamId = @event.StreamId,
            Timestamp = @event.Timestamp,
            Details = $"Order created for {@@event.Data.Amount:C}"
        });
    }

    public void Project(IEvent<OrderCancelled> @event, IDocumentOperations ops)
    {
        ops.Store(new AuditEntry
        {
            Id = Guid.NewGuid(),
            Action = "OrderCancelled",
            StreamId = @event.StreamId,
            Timestamp = @event.Timestamp,
            Details = "Order was cancelled"
        });
    }
}

Registration

opts.Projections.Add<AuditLogProjection>(ProjectionLifecycle.Inline);
// or
opts.Projections.Add<AuditLogProjection>(ProjectionLifecycle.Async);

Use Cases

Event projections are ideal for:

• Audit logs -- Create a record for each significant event
• Search indexes -- Maintain denormalized documents for search
• Notifications -- Create notification records per event
• Cross-cutting concerns -- Track metrics, analytics, or compliance data

Accessing Session Operations

The IDocumentOperations parameter gives you full access to document operations:

public void Project(IEvent<UserDeactivated> @event, IDocumentOperations ops)
{
    // Store new documents
    ops.Store(new DeactivationRecord { ... });

    // Delete documents
    ops.Delete<ActiveUser>(@event.Data.UserId);

    // Patch existing documents
    ops.Patch<UserStats>(@event.Data.UserId)
        .Set(x => x.IsActive, false);
}