Django signal m2m_changed not triggered

Question

I recently started to use signals in my Django project (v. 1.3) and they all work fine except that I just can't figure out why the m2m_changed signal never gets triggered on my model. The Section instance is edited by adding/deleting PageChild inline instances on an django admin form.

I tried to register the callback function either way as described in the documentation, but don't get any result.

Excerpt from my models.py

from django.db import models
from django.db.models.signals import m2m_changed


class Section(models.Model):
    name = models.CharField(unique = True, max_length = 100)
    pages = models.ManyToManyField(Page, through = 'PageChild')

class PageChild(models.Model):
    section = models.ForeignKey(Section)
    page = models.ForeignKey(Page, limit_choices_to = Q(is_template = False, is_background = False))


@receiver(m2m_changed, sender = Section.pages.through)
def m2m(sender, **kwargs):
    print "m2m changed!"

m2m_changed.connect(m2m, sender = Section.pages.through, dispatch_uid = 'foo', weak = False)

Am I missing something obvious?

Answer 1

This is an open bug: https://code.djangoproject.com/ticket/16073

I wasted hours on it this week.

Answer 2

You are connecting it twice, once with m2m_changed.connect and the other time with receiver decorator.

Answer 3

Not sure if it will help, but the following is working for me:

class Flow(models.Model):
    datalist = models.ManyToManyField(Data)

from django.db.models.signals import post_save, pre_delete, m2m_changed

def handle_flow(sender, instance, *args, **kwargs):
    logger.debug("Signal catched !")

m2m_changed.connect(handle_flow, sender=Flow.datalist.through)

Answer 4

I'm not sure if this will help, but are you sure that you should use Sender.pages.through for this special case? perhaps if you tried @reciever(m2m_changed, sender=PageChild)

Note: if you have @reciever, you do not need m2_changed.connect(...) as @reciever already performs the connect operation.