step 12

app/db.py

@@ -0,0 +1,40 @@
+"""Database engine, session factory, and base model.
+
+All app code uses the async SQLAlchemy API.  Alembic migrations use the
+synchronous psycopg2 URL for the CLI (see alembic/env.py).
+
+Usage in routes:
+    from app.db import get_session
+    from sqlalchemy.ext.asyncio import AsyncSession
+
+    async def my_route(db: AsyncSession = Depends(get_session)):
+        result = await db.execute(select(User).where(User.email == email))
+        user = result.scalar_one_or_none()
+"""
+
+from __future__ import annotations
+
+from collections.abc import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+from sqlalchemy.orm import DeclarativeBase
+
+from app.config.settings import settings
+
+engine = create_async_engine(
+    settings.DATABASE_URL,
+    pool_pre_ping=True,
+    echo=settings.ENV == "dev",
+)
+
+async_session = async_sessionmaker(engine, expire_on_commit=False)
+
+
+class Base(DeclarativeBase):
+    """Shared declarative base for all ORM models."""
+
+
+async def get_session() -> AsyncGenerator[AsyncSession, None]:
+    """FastAPI dependency that yields an async DB session per request."""
+    async with async_session() as session:
+        yield session