Automatically run `session.commit()` in session dependency BEFORE returning request

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Example Code

# orm.py
def fastapi_session_scope() -> sqlalchemy.orm.session.Session:
    session = session_maker(bind=engine)
    try:
        yield session
        session.commit()
    except:
        session.rollback()
        raise
    finally:
        session.close()

# routers.py
@router.post("/v1/organizations", response_model=Organization)
def create_org(session: Session = Depends(orm.fastapi_session_scope)):
    org = sqlalc.Organization(name="test")
    session.add(org)
    return org

@router.get("/v1/organizations", response_model=List[Organization])
def get_orgs(session: Session = Depends(orm.fastapi_session_scope)):
    return session.query(sqlalc.Organization).all()

Description

(I realize this is diverging slightly from the SQLAlchemy example, which explicitly uses session.commit() in the body of the function and not in the session dependency.)

I’m creating a row with a POST and then immediately afterwards fetching all rows with a GET. I’ve got session.commit() in the session scope dependency, which is convenient because it runs a commit() at the end no matter what and doesn’t require us to always explicitly commit at the end of an endpoint body.

As written, if an Organization is created via a POST and then all the organizations are immediately afterwards fetched via a GET, the GET will not include the recently created Organization. This is not a JS async issue. What’s happening, as best I can tell, is that the POST starts a transaction, org is returned, and then, before the commit() actually happens, the GET is called and the list of organizations is returned without the organization that was just created (or so it seemed).

t  POST            GET

1  request
2  session.add
3  return
4                  request
5                  session.query
6                  return          # POSTed organization is missing!
7  session.commit
8                  session.commit

The behavior I’d like is for commit() to run BEFORE the value is returned, which would guarantee that the POST and GET could be run back-to-back safely. Is that possible using a FastAPI dependency? Has anyone else run into this issue?

t  POST            GET

1  request
2  session.add
3  session.commit
4  return
5                  request
6                  session.query
7                  session.commit
8                  return

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.59.0

Python Version

3.8.10

Additional Context

No response

[AlexanderPodorov]

Yes, I think this approach should work. And yes, you can create a session in the endpoint function. My 1. and 2. points are related to dependency approach. BTW, as of SQLAlchemy 1.4 we don’t need to write our own context managers for sessions/transactions, it should work out of box. See: https://docs.sqlalchemy.org/en/14/orm/session_transaction.html Example for their site:

Session = sesssionmaker(engine)

with Session() as session:
    with session.begin():
        session.add(some_object)
    # commits

# closes the Session

[AlexanderPodorov]

@anguspmitchell , I think you should avoid dependencies like this:

@contextmanager
def session_scope(expire_on_commit: bool = True) -> sqlalchemy.orm.session.Session:
    orm_session = _orm_session(expire_on_commit=expire_on_commit)
    try:
        yield orm_session
        orm_session.commit()
    except:
        orm_session.rollback()
        raise
    finally:
        orm_session.close()

For those reasons:

1. as @waynerv said, the statement after yield will be executed after the response is returned. So that means, in some circumstances you will send OK response, but commit will actually fail.
2. If you raise HTTPException from endpoint function body, then session will be commited too. This kind of exception won’t be caught by try-except of session_scope function.

P.S. I also tried to use the same approach in the very beginning of my work with FastAPI, but finally I’m confident it’s much better to commit explicitly at the end of endpoint function body.