marten__composite_projection

Follow this guide to create a Composite Projection in Marten (v8.18+).

1. Define the Projection Class

   • Create a class in src/BookStore.ApiService/Projections/
   • Naming: {Summary}Projection (typically a View Projection)
   • Base Class: MultiStreamProjection<T, TId> (usually) or custom.
   • Template: templates/Projection.cs

2. Configure in Marten

   • Open src/BookStore.ApiService/Infrastructure/Extensions/MartenConfigurationExtensions.cs
   • Register using CompositeProjectionFor:
     csharpCopy

     options.Projections.CompositeProjectionFor("GroupDetails", projection =>
     {
         // Stage 1: Basic Projections
         projection.Add<UserProjection>();
         projection.Add<OrderProjection>();
     
         // Stage 2: Dependent Projections (inputs from Stage 1)
         projection.Add<UserDashboardProjection>(2);
     });
     

3. Indexing

   • Configure indexes as usual in ConfigureIndexes.

Related Skills

• /marten__multi_stream_projection: For standard multi-stream aggregates.
• /marten__single_stream_projection: For standard single-stream aggregates.