sqlalchemy flask: AttributeError: 'Session' object has no attribute '_model_changes' on session.commit()

Question

I’ve seen a lot of problems with SessionMaker, but this one is slightly different. Not sure why, but sqlalchemy won’t let my session object commit.

In my app, I have some code that does:

views.py

rec = session.query(Records).filter(Records.id==r).first()
n = rec.checkoutRecord(current_user.id)
session.add(n)
session.commit()

models.py:

class Records(UserMixin, CRUDMixin, Base):
    __table__ = Table('main_records', Base.metadata, autoload=True)


    def checkoutRecord(self,uid):
        self.editing_uid = uid 
        self.date_out = datetime.now()
        return self

    def checkinRecord(self,uid):
        self.editing_uid = uid 
        self.date_in = datetime.now()
        return self

The program craps out on the commit(), giving the above exception. Interestingly, some test code which does not import flask, but does import sqlalchemy works fine and lets me commit without error.

The full stack-trace:

Traceback (most recent call last):
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/flask_login.py", line 663, in decorated_view
    return func(*args, **kwargs)
  File "/Users/bhoward/projects/PeerCoUI/mk2/peercoui/app/records/views.py", line 65, in select_view
    session.commit()
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 149, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 721, in commit
    self.transaction.commit()
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 354, in commit
    self._prepare_impl()
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 323, in _prepare_impl
    self.session.dispatch.before_commit(self.session)
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/sqlalchemy/event.py", line 372, in __call__
    fn(*args, **kw)
  File "/Users/bhoward/Envs/py27/lib/python2.7/site-packages/flask_sqlalchemy/__init__.py", line 162, in session_signal_before_commit
    d = session._model_changes
AttributeError: 'Session' object has no attribute '_model_changes'

Full code for the project is in github: https://github.com/bhoward00/peercoui

Any advice appreciated

Asked By: bhoward

||

Source

Answer 1

Yes this is exactly problem when using flask-sqlalchemy models mixed with pure sqlalchemy session. Thing is that flask-sqlalchemy subclasses the base Session from sqlalchemy and adds some internals one of which is the _model_changes dict. This dict is used for model modification tracking.

So if you want to use flask-sqlalchemy based models with regular sqlalchemy session, one way would be to just add the dict to the session (this is just example code):

def create_session(config):
    engine = create_engine(config['DATABASE_URI'])
    Session = sessionmaker(bind=engine)
    session = Session()
    session._model_changes = {}
    return session

I had the same exact problem as you, so hopefully this should help you.

UPDATE:

There is new version available, which should be fixing this behaviour, quoting the 2.0 docs:

Changed how the builtin signals are subscribed to skip non Flask-SQLAlchemy sessions. This will also fix the attribute error about model changes not existing.

Docs: http://flask-sqlalchemy.pocoo.org/2.0/changelog/#version-2-0

Answered By: jbub

Answer 2

I had the same problem as well and solved it by modifying the _SessionSignalEvents class within __init__.py in flask-sqlalchemy. However, I just noticed that such a fix had already been in place since 8 months on the official repository.

If you encounter a similar problem, I would recommend you to pull the latest version of the project from github (https://github.com/mitsuhiko/flask-sqlalchemy/) since the one currently available through pip install is outdated.

Answered By: olipo186

Answer 3

The SQLAlchemy==1.4.x version is not working for me.

Change to SQLAlchemy==1.3.18 though, and it worked.

How to create such a file to specify the installed versions?

pip3 freeze > requirements.txt
pip3 install flask
pip3 install flask-sqlalchemy

Edit the requirements.txt and the SQLAlchemy entry to Eg SQLAlchemy==1.3.18.

Then, re-run your virtual env. –> flask run

If you have not set a virtual env?

python3 -m venv .venv
source .venv/bin/activate
flask run

Hope that helps !

Answered By: user2060802

Answer 4

I solved this problem do exchange session.exec to session.execute.
I not did deep analysis about this. I think this is due to the fact that SessionMaker returned object, that not original Session

Answered By: Nikita Zakoryuchkin

sqlalchemy flask: AttributeError: 'Session' object has no attribute '_model_changes' on session.commit()

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