SqlAlchemy KeyError: 'post' while using .append()

builtins.KeyError
KeyError: 'post'

Traceback (most recent call last)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/app.py", line 1997, in __call__
return self.wsgi_app(environ, start_response)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/app.py", line 1985, in wsgi_app
response = self.handle_exception(e)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/app.py", line 1540, in handle_exception
reraise(exc_type, exc_value, tb)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/_compat.py", line 33, in reraise
raise value
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/app.py", line 1614, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/app.py", line 1517, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/_compat.py", line 33, in reraise
raise value
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/app.py", line 1612, in full_dispatch_request
rv = self.dispatch_request()
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask/app.py", line 1598, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/flask_login/utils.py", line 261, in decorated_view
return func(*args, **kwargs)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/flask_app/miniblog/blueprints/post.py", line 27, in create
new_post.topics.extend(choice_instances)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/sqlalchemy/orm/collections.py", line 1142, in extend
self.append(value)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/sqlalchemy/orm/collections.py", line 1044, in append
item = __set(self, item, _sa_initiator)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/sqlalchemy/orm/collections.py", line 1016, in __set
item = executor.fire_append_event(item, _sa_initiator)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/sqlalchemy/orm/collections.py", line 680, in fire_append_event
item, initiator)
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/sqlalchemy/orm/attributes.py", line 970, in fire_append_event
initiator or self._append_token or self._init_append_token())
File "/home/jai/Desktop/Pycharm_osrshvhj/osrshvhj_flask/app_env/lib/python3.5/site-packages/sqlalchemy/orm/attributes.py", line 1221, in emit_backref_from_collection_append_event
child_impl = child_state.manager[key].impl
KeyError: 'post'
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