sqlalchemy.orm.exc.DetachedInstanceError in unit test with socketio.test_client

[miguelgrinberg]

Discussed in #1616

^{Originally posted by LostInDarkMath June 27, 2021}
Describe the bug
I just included Flask-SocketIO in my project which is based on Flask-SQLAlchemy and wrote some tests. But I in a simple test I get the following error:

sqlalchemy.orm.exc.DetachedInstanceError: Instance <User at 0x2812f970> is not bound to a Session; attribute refresh operation cannot proceed (Background on this error at: http://sqlalche.me/e/14/bhk3)

The error causing test can be reduced to the following:

def test_receive_notifications_on_new_message_via_websockets(self) -> None:
    socketio_client = self.socketio.test_client(app=self.app, flask_test_client=self.client, headers=self.header())
    self._send_message(sender=self.dummy2, receiver=self.dummy)  # it fails here

If I comment out the the first line that creates the socketio test client, the errors does not appear. This is quite strange because I have no idea why the creation of socketio test client can lead to detached instances.

[miguelgrinberg]

@LostInDarkMath Let me know if this fix addresses the problem for you.

[LostInDarkMath]

Thanks Miguel! This works perfectly fine :)

[LostInDarkMath]

When will you release this fix?