FastAPI Template Skill

FastAPI Template Skill

Generate production-ready FastAPI CRUD modules following a simplified architecture with CRUD helpers.

When to Use This Skill

Use this skill when asked to:

• Create a new FastAPI entity/module (router, CRUD helpers, schemas)
• Add CRUD endpoints to an existing FastAPI application
• Generate reusable query functions following the CRUD helper pattern
• Build REST APIs with SQLAlchemy and Pydantic

Architecture Overview

Primary Pattern (most endpoints):
┌─────────────────────────────────────────────────────────────┐
│                    HTTP Request                              │
└──────────────────────────┬──────────────────────────────────┘
                           │
                           ▼
┌─────────────────────────────────────────────────────────────┐
│  Router (api/routers/setting/{entity}_router.py)            │
│  • Endpoint definitions                                      │
│  • Request/Response validation                               │
│  • session: SessionDep                                       │
│  • Delegates to Service/Controller for most operations       │
└──────────────────────────┬──────────────────────────────────┘
                           │
                           ▼
┌─────────────────────────────────────────────────────────────┐
│  Service/Controller (api/services/ or api/controllers/)      │
│  • Business logic, validation, orchestration                 │
│  • External integrations (AD/LDAP, email, Redis, SMS)       │
│  • Cross-cutting concerns (audit, notifications)             │
└──────────────────────────┬──────────────────────────────────┘
                           │
                           ▼
┌─────────────────────────────────────────────────────────────┐
│  CRUD Helper (api/crud/{entity}.py) — reusable queries       │
│  • Plain async functions (no classes)                        │
│  • Session as first parameter                                │
│  • flush() not commit()                                      │
└──────────────────────────┬──────────────────────────────────┘
                           │
                           ▼
┌─────────────────────────────────────────────────────────────┐
│  Model (db/model.py) + Schemas                               │
│  • SQLModel ORM models                                       │
│  • api/schemas/ (domain) + api/http_schema/ (request/resp)   │
│  • Pydantic DTOs with CamelModel                             │
└─────────────────────────────────────────────────────────────┘

Secondary Pattern (simple entities like schedulers, OUs, domain-users):
┌─────────────────────────────────────────────────────────────┐
│  Router → CRUD helper / Direct queries → Model               │
│  Use only for simple entities with no business logic         │
└─────────────────────────────────────────────────────────────┘

File Structure

api/
├── routers/
│   └── setting/
│       └── {entity}_router.py      # Router with endpoints
├── crud/
│   ├── __init__.py                  # Re-exports all CRUD helpers
│   └── {entity}.py                  # Reusable query functions (3+ uses)
├── services/
│   └── {entity}_service.py          # ONLY for external integrations
├── schemas/
│   ├── _base.py                     # CamelModel base class
│   └── {entity}_schema.py           # Domain schemas
└── http_schema/
    └── {entity}_schema.py           # Request/response Pydantic DTOs

db/
└── model.py                         # SQLModel ORM models

core/
├── dependencies.py                  # SessionDep, CurrentUserDep
├── app_setup/
│   └── routers_group/
│       └── setting_routers.py       # Router registration
└── exceptions.py                    # DetailedHTTPException

Core Principles

1. Single Session Per Request

Every request uses exactly ONE database session via typed dependency:

@router.post("/items")
async def create_item(
    item_create: ItemCreate,
    session: SessionDep,  # Typed dependency - NOT Depends(get_session)
):
    item = Item(**item_create.model_dump())
    session.add(item)
    await session.commit()
    await session.refresh(item)
    return item

2. Session Flow

Simple:
Endpoint (SessionDep) → Direct query OR CRUD helper → Database

Complex:
Endpoint (SessionDep) → Service (receives session) → CRUD helper → Database

3. CamelModel for API Responses

All schemas inherit from CamelModel for automatic snake_case to camelCase conversion:

class ItemResponse(CamelModel):
    item_id: int        # Python: snake_case
    created_at: datetime
    # JSON output: {"itemId": 1, "createdAt": "..."}

4. Domain Exceptions

Use DetailedHTTPException for errors:

from api.exceptions import DetailedHTTPException
from fastapi import status

raise DetailedHTTPException(
    status_code=status.HTTP_404_NOT_FOUND,
    detail=f"Item not found with ID: {item_id}",
)

Generation Order

When creating a new entity, generate files in this order:

1. Model (db/model.py) - Add SQLModel model
2. Schemas (api/schemas/{entity}_schema.py + api/http_schema/{entity}_schema.py) - Pydantic DTOs
3. CRUD helpers (api/crud/{entity}.py) - Only if queries are reused 3+ times
4. Router (api/routers/setting/{entity}_router.py) - API endpoints
5. Register router in core/app_setup/routers_group/setting_routers.py

Quick Reference

CRUD Helper Pattern

• Plain async functions (no classes)
• Session as first parameter
• flush() not commit() - caller commits
• Only create when query is reused 3+ times
• Import as: from api.crud import items as items_crud

Service/Controller Pattern (Primary)

• Most routers delegate to a Service or Controller class
• Business logic, validation, orchestration
• External integrations (AD, email, Redis, SMS)
• Uses CRUD helpers for database access (not repositories)
• Session passed as parameter

Router Pattern

• Use SessionDep typed dependency
• Most routers delegate to Service/Controller
• Direct queries only for simple entities (schedulers, OUs, domain-users)
• Import CRUD helpers for reusable queries
• Path: api/routers/setting/{entity}_router.py
• Registration: core/app_setup/routers_group/setting_routers.py

References

See the references/ directory for detailed patterns:

Core Patterns

• model-pattern.md - SQLAlchemy models
• schema-pattern.md - Pydantic DTO patterns
• crud-helper-pattern.md - Reusable query functions (replaces repository pattern)
• service-pattern.md - External integrations and complex orchestration
• router-pattern.md - API endpoints with SessionDep

Advanced Patterns

• file-upload-pattern.md - File uploads with UploadFile, validation, S3
• testing-pattern.md - pytest fixtures, async tests, dependency overrides
• response-types-pattern.md - HTML, file downloads, streaming, redirects
• middleware-pattern.md - Security headers, correlation ID, timing, logging
• form-data-pattern.md - Form handling, OAuth2 password flow, headers, cookies