Sometimes it is useful to perform an action after something has been added to a many-to-many model in Django. The offical Django documentation is pretty sparse on this topic, so I figured I would document a decent solution. In the top of your models.py file, import Django signals.

Let says your models are setup as follows:

from django.db import models

class Event(models.Model):
name	= models.CharField(max_length=255, help_text="Name of the event")

class Contact(models.Model):
full_name = models.CharField(max_length=255,
help_text="Full ame of the contact")
events = models.ManyToManyField(Event, blank=True, null=True,
help_text="Events this contact is associated with")

And now lets say you want to perform some action, say send an email, everytime a new event is added to the contact.events attribute. The code that will do this looks like this:

def m2m_send_email(signal, sender, **kwargs):
"""
When someone adds/removes someone from the guestlist, you need
to create a RSVP receipt.
"""
from apps.invites.models import RSVPStatistics

contact = kwargs['instance']
action = kwargs['action']
if kwargs['pk_set']:
events = kwargs['model'].objects.filter(pk__in=kwargs['pk_set'])
models.signals.m2m_changed.connect(m2m_send_email, Contact.events.through)
m2m_send_email will get called every time you call contact.events.add(event); contact.save(). You can decide which action you want to support and when to send the email by checking kwargs['action'].