Django's post_save doesn't always

Django’s post_save methods allow execution of custom code on an insert or update of a record after the save method is called.

The serialized object propagates through the signal handlers as the instance parameter which is eventually committed to the database only if no exceptions occur in any of the methods that listen to the signal.

Handlers:

• can be multiple which are synchronously chained

• will executed non-deterministically as per the signal’s receiver registration

• will execute prior to a transaction commit

• will short-circuit execution on failure

Data inconsistencies are caused where proceeding methods in the post_save chain aren’t called after the one that throws an unhandled exception. The database record is not updated.

Additionally, the instance parameter has the most up-to date state which on an update, will revise the stale content that resides in the DB.

To avoid issues:

• Use transaction on_commit to guarantee the data consistency

• Use the instance argument over re-reading it from the DB in handlers

References

• Performing transaction on commit

• Django Signals

• Django’s post_save