I have a Django app that broadcasts changes of some of its models to its clients, to keep them up-to-date.
I have a separate app for that, that binds to the post_save signal of these models and triggers the broadcast.
Problems have come with models with ManyToManyFields. I am not very familiar with Django's handling of this but I understand that these fields are actually updated after the model is saved.
I was able to use the m2m_changed signal to react to the post_* actions and dispatch the broadcast after the object is all up-to-date. However, in the post_save handler, I still need to detect that the m2m field is going to be mutated, to avoid broadcasting incomplete data. How can I detect this , either in the post_save signal handler or the model's save method ? Is there a way to raise a flag on an object when a m2m field is about to be mutated ?
Here's what I've tried :
pre_* actions of the m2m_changed signal to detect incoming mutation of the field but that does not work, because the signal gets fired after post_save, which is too late (data has already been broadcasted).save method to look for changes. This does not work because the fields are not changed and report to be == the initial value ; plus I read on other questions that this practice could cause race conditions.Thanks a lot for the help.
I finally sorted this out. For reference:
TL;DR: The model layer is not the right place to do this ; the view layer is.
I re-implemented the notification system at the view level instead of doing it in the model layer. I made a mixin that adds notification features to my views. Thanks to this solution :