Add post_form_save signal

[James Dabbs]

Several people (see ​stackoverflow, #13950, ​django-developers) have had trouble with / questions about model post_save signals and m2m fields. Personally, I've got a model where the objects almost never change, but every time one is added, I need to run a (relatively expensive) callback function which depends on the entire collection of related objects. Unfortunately, the model's post_save signal fires before the m2m fields are populated, and it's wasteful to run the callback function after every m2m_changed, since only the last change matters.

I propose adding a post_form_save signal to any ModelForm that fires after the form's save_m2m method has been called. That will allow the sort of m2m batch processing that I (and ​this gentleman) are looking for.

[Russell Keith-Magee]

​discussion on Django-dev

Marking wontfix.