Django post_save signal on parent class with multi-table inheritance

Question

In Django, if you have models that use multi-table inheritance, and you define a receiver for a post_save signal on the parent class, does that receiver function get called when an instance of the child class is saved?

Borrowing an example from another question:

class Animal(models.Model):
    category = models.CharField(max_length=20)

class Dog(Animal):
    color = models.CharField(max_length=10)

def echo_category(sender, **kwargs):
    print "category: '%s'" % kwargs['instance'].category

post_save.connect(echo_category, sender=Animal)

If I do:

>>> dog = Dog.objects.get(...)
>>> dog.category = "canine"
>>> dog.save()

Will the echo_category receiver function be called?

score 16 · Answer 1 · answered Jul 08 '14 at 06:13

16

post_save.connect(my_handler, ParentClass)
# connect all subclasses of base content item too
for subclass in ParentClass.__subclasses__():
    post_save.connect(my_handler, subclass)

have a nice day!

answered Jul 08 '14 at 06:13

scythargon

3,363
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2

Where can you put that snippet safely so all subclasses are already registered? – Scott Stafford Mar 08 '16 at 21:42
1

@ScottStafford In the [app ready method](https://docs.djangoproject.com/en/1.11/ref/applications/#django.apps.AppConfig.ready) – Dave Lawrence Oct 16 '17 at 07:57
1

That doesn't work. Subclasses are empty in app ready method. – EugZol Dec 12 '21 at 16:22

score 3 · Accepted Answer · answered Feb 07 '13 at 18:44

3

Check out: https://code.djangoproject.com/ticket/9318 It appears that most propagate the signal to the super in the subclass.

answered Feb 07 '13 at 18:44

bfschott

176
1
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score 1 · Answer 3 · answered Feb 07 '13 at 18:44

1

No, it will not be called. See #9318 in Django trac.

answered Feb 07 '13 at 18:44

Lorin Hochstein

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Benargee · Answer 4 · 2019-05-25T20:02:51.853

I managed to get inherited signal receivers working with the @receiver decorator. See relevant Django documentation

from django.db import models
from django.db.models.signals import post_save
from django.dispatch import receiver

class Animal(models.Model):
    category = models.CharField(max_length=20)

    @receiver(post_save)
    def echo_category(sender, **kwargs):
        print ("category: '%s'" % kwargs['instance'].category)

class Dog(Animal):
    color = models.CharField(max_length=10)

This solution is valid in Python 3.6.8 Django 2.2

When I do this

>>> from myapp.models import Dog
>>> dog = Dog()
>>> dog.category = "canine"
>>> dog.save()
category: 'canine'
>>>

No problems. Everything seems to work from the shell.

Slightly unrelated, but when I edited models through the admin panel There was an issue with it getting called twice so I filtered them by checking the 'created' kwarg. In one call it was false, the other it was true so I just put in a simple if block. Credit for that workaround goes to Pratik Mandrekar and his answer:

from django.db import models
from django.db.models.signals import post_save
from django.dispatch import receiver

class Animal(models.Model):
    category = models.CharField(max_length=20)

    @receiver(post_save)
    def echo_category(sender, **kwargs):
        if not kwargs.get('created'):
            print ("category: '%s'" % kwargs['instance'].category)

class Dog(Animal):
    color = models.CharField(max_length=10)

I have tried this but the receiver seems to get called whenever ANY model gets created for some reason — Mazino, May 03 '20 at 04:34

Django post_save signal on parent class with multi-table inheritance

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