Celery-Flask Demo

Small application written to investigate the problems occurring when using Celery in Flask when Celery is using SQLAlchemy as broker.

Problem

Flask-SQLAlchemy is registering callbacks in SQLAlchemy.Session and it also decorates Session with additional field(s). If at the same time normal SQLAlchemy.Session is used (for instance by Celery) problems occure: callbacks access fields that don't exist in pure Session.

Towards solution

For the application it is possible to fix the problem in a programmatic way by adding a callback that is called before Flask-SQLAlchemy callbacks are called:

@event.listens_for(Session, 'after_attach')
def my_attachement(session, intances):
 if not hasattr(session, '_model_changes'):
   session._model_changes = {}

Unfortunately the same problem occurs while starting the workers

./start_worker.sh

Here (due to high parallelization) the registration of additional callback does not seem to work.

Solution

The development version of the Flask-SQLAlchemy contains improved version of callbacks that check if the Session is instance of Flask-SQLAlchemy decorated session. The installation is possible through:

pip install https://github.com/mitsuhiko/flask-sqlalchemy/archive/master.zip