I have a function that attempts to take a list of usernames, look each one up in a user table, and then add them to a membership table. If even one username is invalid, I want the entire list to be rolled back, including any users that have already been processed. I thought that using sessions was the best way to do this but I'm running into a DetachedInstanceError: DetachedInstanceError: Instance <Organization at 0x7fc35cb5df90> is not bound to a Session; attribute refresh operation cannot proceed Full stack trace is here. The error seems to trigger when I attempt to access the user (model) object that is returned by the query. From my reading I understand that it has something to do with there being multiple sessions, but none of the suggestions I saw on other threads worked for me. Code is below: def add_members_in_bulk(organization_eid, users): """Add users to an organization in bulk - helper function for add_member()""" """Returns "success" on success and id of first failed student on failure""" session = query_session.get_session() session.begin_nested() users = users.split('\n') for u in users: try: user = user_lookup.by_student_id(u) except ObjectNotFoundError: session.rollback() return u if user: membership.add_user_to_organization( user.entity_id, organization_eid, '', [] ) session.flush() session.commit() return 'success' here's the membership.add_user_to_organization: def add_user_to_organization(user_eid, organization_eid, title, tag_ids): """Add a User to an Organization with the given title""" user = user_lookup.by_eid(user_eid) organization = organization_lookup.by_eid(organization_eid) new_membership = OrganizationMembership( organization_eid=organization.entity_id, user_eid=user.entity_id, title=title) new_membership.tags = [get_tag_by_id(tag_id) for tag_id in tag_ids] crud.add(new_membership) and here is the lookup by ID query: def by_student_id(student_id, include_disabled=False): """Get User by RIN""" try: return get_query_set(include_disabled).filter(User.student_id == student_id).one() except NoResultFound: raise ObjectNotFoundError("User with RIN %s does not exist." % student_id)