scaffold-projection

---

name: scaffold-projection description: Create a new Marten read model projection for CQRS queries. Use this when you need to create an optimized read model from event streams.

Follow this guide to create a new Marten projection (read model) for efficient queries in the ApiService.

1. Define the Projection Class

   • Create a class in src/BookStore.ApiService/Projections/
   • Naming: {Resource}Projection (e.g., AuthorProjection)
   • Inheritance: Inherit from appropriate Marten base or use [ViewProjection]
   • Template:

     Copynamespace BookStore.ApiService.Projections;
     
     public class AuthorProjection
     {
         public Guid Id { get; set; }
         public string Name { get; set; } = string.Empty;
         public string Biography { get; set; } = string.Empty;
         public bool Deleted { get; set; }
         public int Version { get; set; }
         public DateTimeOffset CreatedAt { get; set; }
         public DateTimeOffset? UpdatedAt { get; set; }
     
         // For localized content, use Dictionary<string, string>
         public Dictionary<string, string> Biographies { get; set; } = new();
     }
     

2. Implement Create Method

   • Handle the initial event to create the projection
   • Pattern: Static method returning projection instance
   • Example:

     Copypublic static AuthorProjection Create(AuthorCreated @event)
     {
         return new AuthorProjection
         {
             Id = @event.Id,
             Name = @event.Name,
             Biography = @event.Biography,
             CreatedAt = @event.CreatedAt,
             Version = 1
         };
     }
     

3. Implement Apply Methods

   • Handle subsequent events to update the projection
   • Pattern: Instance method, returns void, single event parameter
   • Example:

     Copypublic void Apply(AuthorUpdated @event)
     {
         Name = @event.Name;
         Biography = @event.Biography;
         UpdatedAt = @event.UpdatedAt;
         Version++;
     }
     
     public void Apply(AuthorDeleted @event)
     {
         Deleted = true;
         UpdatedAt = @event.DeletedAt;
         Version++;
     }
     
     public void Apply(AuthorRestored @event)
     {
         Deleted = false;
         UpdatedAt = @event.RestoredAt;
         Version++;
     }
     

4. Configure Marten Projection

   • Open src/BookStore.ApiService/Infrastructure/Extensions/MartenConfigurationExtensions.cs

   • Locate RegisterProjections(StoreOptions options) method

   • Register the projection:

     Copystatic void RegisterProjections(StoreOptions options)
     {
         // Existing configuration...
     
         // Add inline projection
         options.Projections.Add<AuthorProjection>(ProjectionLifecycle.Inline);
     
         // OR for async projection (recommended for production)
         options.Projections.Add<AuthorProjection>(ProjectionLifecycle.Async);
     });
     

     [!NOTE] For Async projections, ensure you also register the ProjectionCommitListener to trigger SSE notifications.

5. Add Indexing (Optional but Recommended)

   • Inside MartenConfigurationExtensions.cs, locate ConfigureIndexes(StoreOptions options):

     Copystatic void ConfigureIndexes(StoreOptions options)
     {
         options.Schema.For<AuthorProjection>()
             .Index(x => x.Name)          // For name searches
             .Index(x => x.Deleted)       // For filtering deleted items
             .GinIndexJsonData();         // For full-text search (if using JSONB)
     }
     

6. Create Queries Using the Projection

   • In endpoints, query the projection (not the aggregate):

     Copypublic static async Task<IResult> GetAuthors(
         IDocumentStore store,
         int page = 1,
         int pageSize = 20,
         CancellationToken cancellationToken = default)
     {
         await using var session = store.QuerySession();
     
         var authors = await session.Query<AuthorProjection>()
             .Where(x => !x.Deleted)
             .OrderBy(x => x.Name)
             .ToPagedListAsync(page, pageSize, cancellationToken);
     
         return Results.Ok(authors);
     }
     

7. Add Caching (Recommended)

   • Wrap projection queries with HybridCache:

     Copypublic static async Task<IResult> GetAuthors(
         IDocumentStore store,
         HybridCache cache,
         int page = 1,
         int pageSize = 20,
         CancellationToken cancellationToken = default)
     {
         var cacheKey = $"authors:page:{page}:size:{pageSize}";
     
         var authors = await cache.GetOrCreateAsync(
             cacheKey,
             async entry =>
             {
                 entry.SetOptions(new HybridCacheEntryOptions
                 {
                     Expiration = TimeSpan.FromMinutes(5),
                     LocalCacheExpiration = TimeSpan.FromMinutes(1)
                 });
     
                 await using var session = store.QuerySession();
                 return await session.Query<AuthorProjection>()
                     .Where(x => !x.Deleted)
                     .OrderBy(x => x.Name)
                     .ToPagedListAsync(page, pageSize, cancellationToken);
             },
             tags: [CacheTags.AuthorList],
             cancellationToken: cancellationToken
         );
     
         return Results.Ok(authors);
     }
     

8. Rebuild Projections (Development)

   • To rebuild all projections from events:

     Copycurl -X POST http://localhost:5000/api/admin/projections/rebuild
     

Projection Types

Inline Projections

• When: Real-time updates required
• Pro: Always up-to-date
• Con: Slower writes (blocks until projection updated)
• Use: Small datasets, critical consistency

Async Projections

• When: High write volume
• Pro: Non-blocking writes, better performance
• Con: Eventually consistent (slight delay)
• Use: Large datasets, acceptable eventual consistency

Localized Content Pattern

For multi-language support:

Copypublic class AuthorProjection
{
    // Store all translations
    public Dictionary<string, string> Biographies { get; set; } = new();

    // Helper method to get localized value
    public string GetBiography(string culture, string defaultCulture = "en")
    {
        return LocalizationHelper.GetLocalizedValue(
            Biographies,
            culture,
            defaultCulture,
            ""
        );
    }
}

Performance Tips

• ✅ Index frequently queried fields (Name, Deleted, etc.)
• ✅ Use GIN indexes for JSONB search (full-text)
• ✅ Limit projection size (only include query-required fields)
• ✅ Use async projections for high-volume events
• ✅ Cache projection queries with appropriate invalidation

Troubleshooting

Projection Not Updating

1. Check Marten configuration includes the projection
2. Verify Apply method signatures match events exactly
3. For async projections, check daemon is running
4. Run projection rebuild: POST /api/admin/projections/rebuild

Missing Data in Queries

1. Ensure projection includes all required fields
2. Check Apply methods handle all relevant events
3. Verify indexes exist for queried fields

Related Skills

Prerequisites:

• /scaffold-aggregate - Ensure events and aggregates exist first
• Events should be defined before creating projections

Next Steps:

• /scaffold-read - Create query endpoints using this projection
• /scaffold-write - Ensure MartenCommitListener sends SSE notifications
• /scaffold-test - Create integration tests
• /verify-feature - Complete verification

Related:

• /debug-cache - If cached projection data is stale

See Also:

• scaffold-aggregate - Event and aggregate patterns
• scaffold-read - Query endpoint implementation
• scaffold-write - SSE notification setup
• marten-guide - Marten event store and projections
• ApiService AGENTS.md - Projection patterns and Marten configuration