Backend Feature

Backend Feature Scaffolding

This skill handles three scenarios:

1. New Entity - Create a complete vertical slice for a new domain entity
2. New Property - Add properties to an existing entity
3. New Feature - Add business operations (commands/queries) to an existing entity

Scenario 1: New Entity

For a new entity named [EntityName] (e.g., Customer, Order, Product), this skill generates:

Domain/
└── [EntityName]s/
    ├── [EntityName].cs                 # Rich domain entity
    ├── DomainEvents/
    │   ├── [EntityName]Created.cs
    │   ├── [EntityName]Updated.cs
    │   └── [EntityName]Deleted.cs
    ├── Dtos/
    │   ├── [EntityName]Dto.cs
    │   ├── [EntityName]ForCreationDto.cs
    │   ├── [EntityName]ForUpdateDto.cs
    │   └── [EntityName]ParametersDto.cs
    ├── Features/
    │   ├── Add[EntityName].cs
    │   ├── Update[EntityName].cs
    │   ├── Delete[EntityName].cs
    │   ├── Get[EntityName].cs
    │   └── Get[EntityName]List.cs
    ├── Mappings/
    │   └── [EntityName]Mapper.cs
    └── Models/
        ├── [EntityName]ForCreation.cs
        └── [EntityName]ForUpdate.cs

Databases/
└── EntityConfigurations/
    └── [EntityName]Configuration.cs    # EF Core entity configuration

Controllers/
└── v1/
    └── [EntityName]sController.cs

Scenario 2: New Property on Existing Entity

When adding properties to an existing entity, update these files:

Scenario 3: New Feature on Existing Entity

When adding a business operation to an existing entity:

Usage Examples

New Entity

Create a backend feature for managing Products with name, price, and category

New Property on Existing Entity

Add an email property to the Customer entity

Add a status property with Draft, Active, and Archived states to the Product entity

New Feature on Existing Entity

Add a Submit feature to the Order entity that changes status and validates requirements

Add a GetOrdersByCustomer query that returns orders for a specific customer

Process: New Entity

1. Gather Requirements

   • Entity name (PascalCase, singular: Customer, Order, Product)
   • Properties and their types
   • Any special business rules or value objects needed

2. Generate Domain Entity

   • Create rich domain entity following DDD principles
   • Private setters, factory methods, encapsulated behavior
   • Value objects for complex concepts
   • See: .claude/rules/backend/working-with-domain-entities.md

3. Generate Domain Events

   • Created, Updated, Deleted events
   • Additional business-specific events as needed
   • See: .claude/rules/backend/domain-events.md

4. Generate DTOs and Models

   • Read DTO for responses
   • Creation/Update DTOs for requests
   • Parameters DTO for list queries
   • Internal models for domain method contracts
   • See: .claude/rules/backend/dtos-and-mappings.md

5. Generate Mapperly Mapper

   • Entity to DTO mappings
   • DTO to internal model mappings
   • IQueryable projection support
   • See: .claude/rules/backend/dtos-and-mappings.md

6. Generate CQRS Features

   • Add, Update, Delete commands
   • Get single and Get list queries
   • MediatR handlers with proper patterns
   • See: .claude/rules/backend/features-and-cqrs.md

7. Generate Controller

   • Thin controller delegating to MediatR
   • Proper API versioning
   • OpenAPI documentation
   • See: .claude/rules/backend/controllers.md

8. Generate Entity Configuration

   • Create IEntityTypeConfiguration<T> class
   • Configure property constraints and indexes
   • Configure value object mappings with OwnsOne
   • See: .claude/rules/backend/entity-configurations.md

9. Update Infrastructure

   • Add DbSet to AppDbContext
   • Ensure ApplyConfigurationsFromAssembly is called in OnModelCreating

Process: New Property

1. Read Existing Entity

   • Understand current entity structure and patterns
   • Identify if property is a simple type or value object

2. Add Property to Domain Entity

   // Simple property
   public string PhoneNumber { get; private set; }
   
   // Value object property
   public EmailAddress Email { get; private set; } = default!;
   

3. Update Factory Method (if set at creation)

   public static Customer Create(CustomerForCreation forCreation)
   {
       var entity = new Customer
       {
           // ... existing properties
           PhoneNumber = forCreation.PhoneNumber,  // Add new property
       };
       // ...
   }
   

4. Update Update Method (if updatable)

   public Customer Update(CustomerForUpdate forUpdate)
   {
       // ... existing updates
       PhoneNumber = forUpdate.PhoneNumber;  // Add new property
       // ...
   }
   

5. Update Internal Models

   • Add to [EntityName]ForCreation.cs if set at creation
   • Add to [EntityName]ForUpdate.cs if updatable

6. Update DTOs

   • Add to [EntityName]Dto.cs for responses
   • Add to [EntityName]ForCreationDto.cs if set at creation
   • Add to [EntityName]ForUpdateDto.cs if updatable

7. Update Mapper (for value objects)

   // Add custom mapping for value objects
   private static string? MapEmail(EmailAddress? email) => email?.Value;
   

8. Update Entity Configuration

   // Simple property
   builder.Property(e => e.PhoneNumber).HasMaxLength(20);
   
   // Value object
   builder.OwnsOne(e => e.Email, email =>
   {
       email.Property(e => e.Value).HasColumnName("email").HasMaxLength(320);
   });
   

9. Create Migration

   dotnet ef migrations add Add[PropertyName]To[EntityName]
   

Process: New Feature

1. Read Existing Entity

   • Understand current entity structure
   • Identify what state changes are needed
   • Determine guard conditions

2. Add Domain Method to Entity

   public Order Submit()
   {
       GuardIfNotDraft("Cannot submit");
   
       Status = OrderStatus.Submitted();
   
       QueueDomainEvent(new OrderSubmitted
       {
           OrderId = Id,
           SubmittedAt = DateTimeOffset.UtcNow
       });
   
       return this;
   }
   

3. Create Domain Event (if needed)

   // DomainEvents/OrderSubmitted.cs
   public sealed record OrderSubmitted : IDomainEvent
   {
       public required Guid OrderId { get; init; }
       public required DateTimeOffset SubmittedAt { get; init; }
   }
   

4. Create Feature (Command or Query)

   // Features/SubmitOrder.cs
   public static class SubmitOrder
   {
       public sealed record Command(Guid Id) : IRequest<OrderDto>;
   
       public sealed class Handler(AppDbContext dbContext) : IRequestHandler<Command, OrderDto>
       {
           public async Task<OrderDto> Handle(Command request, CancellationToken ct)
           {
               var order = await dbContext.Orders.GetById(request.Id, ct);
   
               order.Submit();
   
               await dbContext.SaveChangesAsync(ct);
   
               return order.ToOrderDto();
           }
       }
   }
   

5. Add Endpoint to Controller

   /// <summary>
   /// Submits an order for processing.
   /// </summary>
   [Authorize]
   [HttpPost("{id:guid}/submit", Name = "SubmitOrder")]
   [ProducesResponseType(typeof(OrderDto), StatusCodes.Status200OK)]
   [ProducesResponseType(StatusCodes.Status404NotFound)]
   [ProducesResponseType(StatusCodes.Status400BadRequest)]
   public async Task<ActionResult<OrderDto>> SubmitOrder(Guid id)
   {
       var command = new SubmitOrder.Command(id);
       var result = await mediator.Send(command);
       return Ok(result);
   }
   

6. Create Additional DTOs (if needed)

   • Request DTO if the action needs input parameters
   • Custom response DTO if different from standard entity DTO

Important Guidelines

Domain Entity Design

• Use private setters for all properties
• Create static factory method Create() for entity creation
• Use Update() method for modifications
• Implement guard methods for business rules
• Use value objects for complex domain concepts
• Queue domain events on state changes

Property Naming

• Entity properties: PascalCase (FirstName, Email)
• DTO properties: PascalCase with { get; init; }
• Route parameters: camelCase in URLs, PascalCase in code

Value Objects to Consider

• EmailAddress - validated email
• PhoneNumber - formatted phone
• Money - currency with amount
• Address - street, city, state, zip
• Status enums with SmartEnum pattern

Common Patterns

Status with Behavior:

public class OrderStatus : ValueObject
{
    private OrderStatusEnum _status;

    public bool IsFinalState() => _status.IsFinalState();
    public bool CanBeCancelled() => !IsFinalState();

    public static OrderStatus Draft() => new("Draft");
    public static OrderStatus Submitted() => new("Submitted");
}

Encapsulated Collections:

private readonly List<LineItem> _lineItems = [];
public IReadOnlyCollection<LineItem> LineItems => _lineItems.AsReadOnly();

public LineItem AddLineItem(Product product, int quantity)
{
    var item = LineItem.Create(product, quantity);
    _lineItems.Add(item);
    return item;
}

Checklists

New Entity Checklist

• Review generated entity for business logic completeness
• Add DbSet to AppDbContext.cs
• Verify entity configuration is in Databases/EntityConfigurations/ and properly applied
• Install MediatR if not already present
• Install Mapperly if not already present
• Run dotnet build to verify compilation
• Add any needed FluentValidation validators
• Consider what additional business methods the entity needs
• Run dotnet ef migrations add Add[EntityName] to create migration

New Property Checklist

• Property added with private setter in entity
• Factory method updated (if set at creation)
• Update method updated (if updatable)
• Internal models updated (ForCreation/ForUpdate)
• DTOs updated (Dto, ForCreationDto, ForUpdateDto as needed)
• Mapper updated (custom mapping for value objects)
• Entity configuration updated (constraints, value object mapping)
• Run dotnet build to verify compilation
• Run dotnet ef migrations add Add[PropertyName]To[EntityName]

New Feature Checklist

• Domain method added to entity with proper guards
• Domain event created (if state change is significant)
• Feature file created with Command/Query and Handler
• Controller endpoint added with proper attributes
• Additional DTOs created if needed
• Run dotnet build to verify compilation

Reference Files

The following rules provide detailed patterns:

• .claude/rules/backend/working-with-domain-entities.md - Rich domain entities
• .claude/rules/backend/features-and-cqrs.md - CQRS with MediatR
• .claude/rules/backend/dtos-and-mappings.md - DTOs and Mapperly
• .claude/rules/backend/controllers.md - API controllers
• .claude/rules/backend/domain-events.md - Domain events
• .claude/rules/backend/entity-configurations.md - EF Core entity configurations