FastAPI Presentation Layer (DDD / Onion Architecture)

This skill focuses on the Presentation layer only: FastAPI routes/handlers, Pydantic schemas, and HTTP error mapping. It assumes you already have Domain + UseCase layers and repository wiring.

Non-negotiables (Onion rule)

• Presentation is the outermost layer: it can depend on UseCase and Domain types, but Domain must not depend on FastAPI/Pydantic.
• Keep business rules inside Domain/UseCase. Presentation does:
  • request parsing/validation (shape, basic constraints)
  • conversion primitives → Value Objects
  • calling usecase.execute(...)
  • mapping Domain exceptions → HTTPException
  • conversion Entity → response schema

Recommended structure (per aggregate)