I found the root cause while trying to narrow down the code that caused the exception. I placed the same attribute access code at different places after session close and found that it definitely doesn’t cause any issue immediately after the close of query session. It turns out the problem starts appearing after closing a fresh session that is opened to update the object. Once I understood that the state of the object is unusable after a session close, I was able to find this thread that discussed this same issue. Two solutions that come out of the thread are:
- Keep a session open (which is obvious)
- Specify
expire_on_commit=Falsetosessionmaker().
The 3rd option is to manually set expire_on_commit to False on the session once it is created, something like: session.expire_on_commit = False. I verified that this solves my issue.