id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
6778	dispatch signal needed on many to many relationship alterations (add, delete)	xore.ander@…	Andrew Gibson	"When a model has a relation added, no signal is sent. 

This breaks functionality where the model is internally caching processed relational data, and the model becomes associated with new (or disassociated with) items. There is no method to inform the original model that it's cache is stale.

it would seem appropriate that all functions with the property 'alters_data' (originally intended for template use) would have pre/post signals.
(django/db/models/fields/related.py, django/contrib/contenttypes/generic.py:  add, create, remove, clear )

to give a trivial example:
{{{
class Attendee(models.Model):
	name = models.CharField(maxlength=255)

class Conference(models.Model):
	name = models.CharField(maxlength=255)
	attendees = models.ManyToManyField(Attendee, related_name='conferences')
	def num_attendees(self):
		if not hasattr(self,""attendee_count""):
			setattr(self,""attendee_count"",len(self.attendees.all()))
		return self.attendee_count
	def stale_cache(self):
		if hasattr(self,""attendee_count""):
			delattr(self,""attendee_count"")
}}}


(generally, i'm looking to do more complex processes than take the len() of a queryset...)
{{{
a,b,c = Attendee(name=""Alice""), Attendee(name=""Bob""), Conference(name=""PyCon"")
for i in (a,b,c):
	i.save()

c.attendees.add(a)
c.num_attendees()
c.attendees.add(b)
c.num_attendees()
}}}

obviously calls to stale_cache() would fix the problem, but the application-level programmer shouldn't necessarily have to deal with cache maintenance. (DRY... someone's bound to forget to update cache somewhere), it would be better of there is a signal to do this on relational updates.

Enabling this should be fairly straightforward:

* add appropriate signals to django.db.models.signals

* call the dispatcher from appropriate functions in the relational model managers, etc"		closed	Core (Other)	dev		duplicate	dispatcher relation m2m	George Song Christian Schilling rvdrijst	Accepted	1	1	0	1	0	0