sqlalchemy: Error with commit on session - 'SessionMaker' object has no attribute '_model_changes'

Question

I'm new to SqlAlchemy. We were working primarily with Flask, but in a particular case I needed a manual database connection. So I launched a new db connection with something like this:

write_engine = create_engine("mysql://user:pass@localhost/db?charset=utf8")
write_session = scoped_session(sessionmaker(autocommit=False,
    autoflush=False,bind=write_engine))

nlabel = write_session.query(Label).filter(Label.id==label.id).first() # Works
#Later in code
ms = Message("Some message")
write_session.add(ms) # Works fine
write_session.commit() # Errors out

Error looks like "AttributeError: 'SessionMaker' object has no attribute '_model_changes'"

What am I doing wrong?

What does the Message model look like? If you are using the Flask-Sqlalchemy extension, this may come in handy: github.com/mitsuhiko/flask-sqlalchemy/pull/89 — aezell
– aezell, Commented Sep 18, 2012 at 19:11
write_session should really be an instance of Session, not a SessionMaker — Damien
– Damien, Commented Dec 3, 2014 at 14:40

bm1362 · Accepted Answer · 2012-10-29 17:05:42Z

3

From the documentation I think you might be missing the initialization of the Session object.

Try:

Session = scoped_session(sessionmaker(autocommit=False, autoflush=False,bind=write_engine))
write_session = Session()

It's a shot in the dark- I'm not intimately familiar with SQLAlchemy. Best of luck!

answered Oct 29, 2012 at 17:05

bm1362

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user2341072 · Accepted Answer · 2014-05-13 21:10:47Z

2

Your issue is that you are missing this line:

db_session._model_changes = {}

answered May 13, 2014 at 21:10

user2341072

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2 Comments

Our Man in Bananas Over a year ago

please add some explanation of your solution and how it resolves the problem

chfw Over a year ago

@Philip, because flask attaches a static method for the session. Hence, before commit it checks _model_changes. See flask_sqlclchemy's init.py. Here is the function:def session_signal_before_commit(session): d = session._model_changes if d: before_models_committed.send(session.app, changes=d.values())

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