Django Recipes: Manipulating a model’s m2m from within a signal handler

gistfile1.py

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

def save_handler(sender, instance, *args, **kwargs):
    m2m_changed.connect(m2m_handler, sender=sender.m2mfield.through, weak=False)

    
def m2m_handler(sender, instance, action, *args, **kwargs):
    if action =='post_clear':
        succesfully_manipulate_m2m(instance)

pre_save.connect(save_handler, sender=YouModel, weak=False)

http://polyglotfactotum.wordpress.com/2013/09/06/django-recipes-manipulating-a-models-m2m-from-within-a-signal-handler/

problem:
When you manipulate the m2m of a model within a post or pre_save signal receiver, your changes get wiped out in the subsequent ‘clearing’ of the m2m by Django.

solution:
In you post or pre_save signal handler, register another handler to the m2m_changed signal on the m2m intermediary model of the model whose m2m you want to update.

Please note that this second handler will receive several m2m_changed signals, and it is key to test for the value of the ‘action’ arguments passed along with them.

Within this second handler, check for the ‘post_clear’ action. When you receive a signal with the post_clear action, the m2m has been cleared by Django and you have a chance to successfully manipulate it.