How to connect FastAPI with PostgreSQL using SQLAlchemy and async sessions?

Question

I’m building a small project using FastAPI + PostgreSQL and I want to use async SQLAlchemy sessions for queries. I’ve seen examples with SessionLocal, but I’m confused about how to properly set up:

engine (async)

session dependency injection

creating models and migrations (with Alembic)

Here’s my minimal setup so far:

from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine
from sqlalchemy.orm import sessionmaker

DATABASE_URL = "postgresql+asyncpg://user:password@localhost/db"

engine = create_async_engine(DATABASE_URL, echo=True)
SessionLocal = sessionmaker(engine, expire_on_commit=False, class_=AsyncSession)

Girum Senay · Accepted Answer · 2025-09-25 07:09:11Z

you are already on the right track. you are only missing how you wire the session into FastAPI and how migrations work with async.

For the Engine + Session you can create them like

from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine
from sqlalchemy.orm import sessionmaker

DATABASE_URL = "postgresql+asyncpg://user:password@localhost/db"

engine = create_async_engine(DATABASE_URL, echo=True)

SessionLocal = sessionmaker(
    bind=engine,
    class_=AsyncSession,
    expire_on_commit=False,
)

after that you add the session dependency

from fastapi import Depends

async def get_session() -> AsyncSession:
    async with SessionLocal() as session:
        yield session

Then in your routes use them for example:

@app.get("/users")
async def list_users(session: AsyncSession = Depends(get_session)):
    result = await session.execute(select(User))
    return result.scalars().all()

Alembic migrations usually trips people up. Alembic itself runs synchronously, so just use the normal Postgres URL (postgresql://...) in alembic.ini. so in your alembic env file u can use

from app.models import Base  

target_metadata = Base.metadata

With that in place, when you run: alembic revision --autogenerate -m "add users table" Alembic will compare your current database schema against the Base.metadata from your models and generate the migration script.

Collectives™ on Stack Overflow

How to connect FastAPI with PostgreSQL using SQLAlchemy and async sessions?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related