如果引发异常，如何使 Flask-SQLAlchemy 自动回滚会话？

Question

I'd like to setup an app built with Flask-SQLAlchemy to rollback all changes done to the database if the view raises an exception that bubbles outside the view code (ie not catched inside).

I'd like it to work even if some objects are flushed to the database in subtransactions, either automatically or directly via session.commit() .

Something similar to Django's transaction request wrapping .

Answer 1

you can do something like this:

@app.teardown_request
def teardown_request(exception):
    if exception:
        db.session.rollback()
    db.session.remove()

Have a look here for teardown_request info. You might need to set the PRESERVE_CONTEXT_ON_EXCEPTION config variable if you are in debug mode.

Answer 2

You could include the rollback in a custom error handler...

@app.errorhandler(500)
def internal_error(error):
    db.session.rollback()
    return render_template('500.html'), 500

See here under the Custom Error Pages section.

Answer 3

The most reliable solution that has worked for me, inspired by dduffy's answer , is to attach an errorhandler that specifically catches SQLAlchemy exceptions and issue a rollback.

from sqlalchemy import exc
@app.errorhandler(exc.SQLAlchemyError)
def handle_db_exceptions(error):
    #log the error: app.logger.error(error)
    db.session.rollback()

The @app.teardown_request hook approach didn't seem to fix the db connection that was in a bad state in time before it was reused by later requests pulling it from the db connection pool. The errorhandler approach catches the connection before it is reused, and by limiting it in scope to the class of errors raised by SQLAlchemy it feels more precise.