FastAPI Structure Guide (2026 Optimized Edition)

Intent

Use this guide whenever generating code for a FastAPI project, specifically when:

1. Scaffolding a brand new project.
2. Adding a new feature (e.g., "Add an Order module").
3. Refactoring existing code to meet 2026 clean architecture standards.

You must strictly adhere to the Core Principles, Project Structure, Development Workflow, and Coding Rules defined below.

I. Core Principles

Before writing any code, follow these six guiding principles:

1. Separation of Concerns (Clean Architecture)

   • API Layer: Only reception, validation, HTTP concerns.
   • Service Layer: Pure business logic and orchestration.
   • Repository Layer: All data access (SQL, caching, external services).
   • DB/Model Layer: Data definition (SQLModel).
   • Rule: Never put business logic or raw SQL in API routes or services.

2. Full Async First

   • All routes, services, repositories must be async def by default.
   • Use async_sessionmaker + await everywhere.
   • Only use sync when absolutely necessary (e.g., legacy libs).

3. Repository Pattern + Dependency Injection

   • Services never touch Session directly.
   • Use FastAPI Depends + Annotated for injection.
   • Flow: DB Session → Repository → Service → API Route.

4. Mandatory Use of SQLModel

   • All database models and base schemas must use SQLModel (Pydantic v2 + SQLAlchemy 2.0).
   • One class serves as both DB table (table=True) and API schema base.
   • Never use raw SQLAlchemy + separate Pydantic models.
   • Always consult references/sqlmodel-reference.md for exact syntax, schema variants, relationships, and FastAPI integration patterns.

5. Config Centralization

   • All config via Pydantic Settings v2 (BaseSettings).
   • Never hardcode secrets, URLs, or keys.

6. Mirrored & Layered Testing

   • tests/ mirrors app/ 1:1.
   • Separate unit/, integration/.
   • Use SQLite in-memory + dependency overrides + pytest-asyncio.

II. Recommended Project Structure (2026 Standard)

my-fastapi-project/
├── app/                          # Core Application
│   ├── __init__.py
│   ├── main.py                   # App factory + lifespan
│   ├── api/                      # 🌐 API Layer
│   │   ├── __init__.py
│   │   └── v1/
│   │       ├── __init__.py
│   │       ├── api.py            # Router aggregation
│   │       └── endpoints/
│   │           ├── __init__.py
│   │           ├── users.py
│   │           └── items.py
│   ├── core/                     # ⚙️ Cross-cutting
│   │   ├── __init__.py
│   │   ├── config.py             # Settings
│   │   ├── logging.py
│   │   ├── security.py
│   │   └── exceptions.py         # Custom HTTP exceptions
│   ├── db/                       # 🗄️ Database
│   │   ├── __init__.py
│   │   ├── session.py            # async_sessionmaker
│   │   ├── models.py             # SQLModel definitions (table=True)
│   │   └── alembic/              # Migrations
│   ├── schemas/                  # 📝 API Schemas (DTOs)
│   │   ├── __init__.py
│   │   └── user.py               # UserCreate, UserResponse, etc.
│   ├── repositories/             # 🗃️ Data Access Layer (NEW)
│   │   ├── __init__.py
│   │   ├── base.py
│   │   ├── user_repository.py
│   │   └── item_repository.py
│   ├── services/                 # 🧠 Business Logic
│   │   ├── __init__.py
│   │   ├── base.py
│   │   ├── user_service.py
│   │   └── item_service.py
│   └── dependencies.py           # Centralized Depends functions
├── tests/                        # ✅ Tests (mirrored)
│   ├── __init__.py
│   ├── conftest.py
│   ├── unit/
│   └── integration/
│       └── api/
│           └── v1/
│               └── endpoints/
│                   └── test_users.py
├── .env                          # Gitignored
├── .env.example
├── .gitignore
├── docker-compose.yaml
├── Dockerfile
├── pyproject.toml                # uv + ruff + pyright + pytest-asyncio
└── README.md

Directory Responsibilities (Updated)

• app/schemas/: API input/output models (inherits from SQLModel when possible).
• app/repositories/: All DB operations, caching, external API calls. Thin wrapper around SQLModel.
• app/services/: Business rules, orchestration, validation. Depends on repositories.
• app/db/models.py: SQLModel classes with table=True.
• app/core/exceptions.py: Custom exceptions + HTTPException handlers.

III. Creation Rules (Development Workflow)

When adding a new feature, follow these 6 Standard Steps in strict order:

Before Step A: Read references/sqlmodel-reference.md.

Step A: Database Model
Add SQLModel class in app/db/models.py (or split file if large).

Step B: API Schemas
Create app/schemas/resource.py (Create/Update/Response variants).

Step C: Repository
Create app/repositories/resource_repository.py (CRUD methods).

Step D: Service
Create app/services/resource_service.py (business logic using repository).

Step E: API Endpoint
Create app/api/v1/endpoints/resource.py (thin routes).

Step F: Registration & Testing

1. Register router in app/api/v1/api.py.
2. Write mirrored tests in tests/integration/.

IV. Coding Rules (2026 Modern Examples)

Rule 0: SQLModel Strict Compliance

All SQLModel code must exactly match patterns in references/sqlmodel-reference.md. Any deviation must be rejected and corrected.

Rule 1: API Routes Must Be Thin & Async

# ✅ Correct
@router.post("/users", response_model=UserResponse)
async def create_user(
    user_in: UserCreate,
    service: UserService = Depends(get_user_service),
):
    return await service.create_user(user_in)

Rule 2: Repository Pattern (Data Access)

# app/repositories/user_repository.py
from sqlmodel.ext.async_session import AsyncSession
from sqlmodel import select
from app.db.models import User

class UserRepository:
    def __init__(self, session: AsyncSession):
        self.session = session

    async def create(self, user: User) -> User:
        self.session.add(user)
        await self.session.commit()
        await self.session.refresh(user)
        return user

    async def get_by_email(self, email: str) -> User | None:
        statement = select(User).where(User.email == email)
        result = await self.session.exec(statement)
        return result.first()

Rule 3: Service Layer (Business Logic)

# app/services/user_service.py
class UserService:
    def __init__(self, repo: UserRepository):
        self.repo = repo

    async def create_user(self, data: UserCreate) -> User:
        # Business rules here
        if await self.repo.get_by_email(data.email):
            raise UserAlreadyExists()
        user = User(**data.model_dump(exclude={"password"}))
        # hash password etc.
        return await self.repo.create(user)

Rule 4: Dependencies (centralized)

# app/dependencies.py
from fastapi import Depends
from sqlmodel.ext.async_session import async_sessionmaker

async def get_db() -> AsyncSession:
    async with sessionmaker() as session:   # from db/session.py
        yield session

def get_user_repository(db: AsyncSession = Depends(get_db)) -> UserRepository:
    return UserRepository(db)

def get_user_service(repo: UserRepository = Depends(get_user_repository)) -> UserService:
    return UserService(repo)

Rule 5: SQLModel Usage

# app/db/models.py
from sqlmodel import SQLModel, Field

class User(SQLModel, table=True):
    id: int | None = Field(default=None, primary_key=True)
    email: str = Field(index=True, unique=True)
    hashed_password: str