How to use Django model inheritance with signals?

It’s also possible to use content types to discover subclasses – assuming you have the base class and subclasses packaged in the same app. Something like this would work:

from django.contrib.contenttypes.models import ContentType
content_types = ContentType.objects.filter(app_label="your_app")
for content_type in content_types:
    model = content_type.model_class()
    post_save.connect(update_attachment_count_on_save, sender=model)

You could register the connection handler without sender specified. And filter the needed models inside it.

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


@receiver(post_save)
def my_handler(sender, **kwargs):
    # Returns false if 'sender' is NOT a subclass of AbstractModel
    if not issubclass(sender, AbstractModel):
       return
    ...

Ref: https://groups.google.com/d/msg/django-users/E_u9pHIkiI0/YgzA1p8XaSMJ

This solution resolves the problem when not all modules imported into memory.

def inherited_receiver(signal, sender, **kwargs):
    """
    Decorator connect receivers and all receiver's subclasses to signals.

        @inherited_receiver(post_save, sender=MyModel)
        def signal_receiver(sender, **kwargs):
            ...

    """
    parent_cls = sender

    def wrapper(func):
        def childs_receiver(sender, **kw):
            """
            the receiver detect that func will execute for child 
            (and same parent) classes only.
            """
            child_cls = sender
            if issubclass(child_cls, parent_cls):
                func(sender=child_cls, **kw)

        signal.connect(childs_receiver, **kwargs)
        return childs_receiver
    return wrapper

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!

The simplest solution is to not restrict on the sender, but to check in the signal handler whether the respective instance is a subclass:

@receiver(post_save)
def update_attachment_count_on_save(sender, instance, **kwargs):
    if isinstance(instance, WorkAttachment):
        ...

However, this may incur a significant performance overhead as every time any model is saved, the above function is called.

I think I’ve found the most Django-way of doing this: Recent versions of Django suggest to connect signal handlers in a file called signals.py. Here’s the necessary wiring code:

your_app/__init__.py:

default_app_config = 'your_app.apps.YourAppConfig'

your_app/apps.py:

import django.apps

class YourAppConfig(django.apps.AppConfig):
    name = 'your_app'
    def ready(self):
        import your_app.signals

your_app/signals.py:

def get_subclasses(cls):
    result = [cls]
    classes_to_inspect = [cls]
    while classes_to_inspect:
        class_to_inspect = classes_to_inspect.pop()
        for subclass in class_to_inspect.__subclasses__():
            if subclass not in result:
                result.append(subclass)
                classes_to_inspect.append(subclass)
    return result

def update_attachment_count_on_save(sender, instance, **kwargs):
    instance.work.attachment_count += 1
    instance.work.save()

for subclass in get_subclasses(WorkAttachment):
    post_save.connect(update_attachment_count_on_save, subclass)

I think this works for all subclasses, because they will all be loaded by the time YourAppConfig.ready is called (and thus signals is imported).

Michael Herrmann’s solution is definitively the most Django-way of doing this.
And yes it works for all subclasses as they are loaded at the ready() call.

I would like to contribute with the documentation references :

In practice, signal handlers are usually defined in a signals submodule of the application they relate to. Signal receivers are connected in the ready() method of your application configuration class. If you’re using the receiver() decorator, simply import the signals submodule inside ready().

https://docs.djangoproject.com/en/dev/topics/signals/#connecting-receiver-functions

And add a warning :

The ready() method may be executed more than once during testing, so you may want to guard your signals from duplication, especially if you’re planning to send them within tests.

https://docs.djangoproject.com/en/dev/topics/signals/#connecting-receiver-functions

So you might want to prevent duplicate signals with a dispatch_uid parameter on the connect function.

post_save.connect(my_callback, dispatch_uid="my_unique_identifier")

In this context I’ll do :

for subclass in get_subclasses(WorkAttachment):
    post_save.connect(update_attachment_count_on_save, subclass, dispatch_uid=subclass.__name__)

https://docs.djangoproject.com/en/dev/topics/signals/#preventing-duplicate-signals

I just did this using python’s (relatively) new __init_subclass__ method:

from django.db import models

def perform_on_save(*args, **kw):
    print("Doing something important after saving.")

class ParentClass(models.Model):
    class Meta:
        abstract = True

    @classmethod
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        models.signals.post_save.connect(perform_on_save, sender=cls)

class MySubclass(ParentClass):
    pass  # signal automatically gets connected.

This requires django 2.1 and python 3.6 or better. Note that the @classmethod line seems to be required when working with the django model and associated metaclass even though it’s not required according to the official python docs.

In addition to @clwainwright answer, I configured his answer to instead work for the m2m_changed signal. I had to post it as an answer for the code formatting to make sense:

@classmethod
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        for m2m_field in cls._meta.many_to_many:
            if hasattr(cls, m2m_field.attname) and hasattr(getattr(cls, m2m_field.attname), 'through'):
                models.signals.m2m_changed.connect(m2m_changed_receiver, weak=False, sender=getattr(cls, m2m_field.attname).through)

It does a couple of checks to ensure it doesn’t break if anything changes in future Django versions.