Preferred way to commit/rollback

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When read about FastAPI’s dependency system with yield state, its pretty tempting to use database transactions with them like so:

async def get_db(request: Request):
    """ database: Database = Depends(get_db) """
    database: Database = request.app.state.database
    async with database.transaction():
        yield database

But this is incorrect way to do it because if transaction would not commit (teardown code in async with raises error) then there is no way to change response.

So this case about transactions would be good to note in docs for dependencies with yield statement (that this is incorrect way). Maybe add link to starlette docs about transactions: https://www.starlette.io/database/#transactions.

[lephuongbg]

You could use middleware to have every endpoint automatically wrapped in transaction like us:

async def automatic_transaction(request: Request, call_next):
    """Wrap all data-modifying requests in transaction"""
    from models import db # We use peewee

    if request.method.lower() in ("post", "put", "patch", "delete"):
        with db.atomic() as txn: # Auto-rollback transaction on server exception
            response = await call_next(request)
            if txn.status_code >= 400: # Auto-rollback transaction on handled exception
                txn.rollback()
    else:
        response = await call_next(request)

    return response

app.add_middleware(BaseHTTPMiddleware, dispatch=automatic_transaction)

[artslob]

Good. I vote for 2 things:

1. Add a warning to dependency injection docs that its bad place to make a transaction
2. Maybe add link to Starlette docs about transactions