How to use Django model inheritance with signals?

Question

I have a few model inheritance levels in Django:

class WorkAttachment(models.Model):
    \"\"\" Abstract class that holds all fields that are required in eac         


        

Answer 1

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

Answer 2

You could try something like:

model_classes = [WorkAttachment, WorkAttachmentFileBased, WorkAttachmentPicture, ...]

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

for model_class in model_classes:
    post_save.connect(update_attachment_count_on_save, 
                      sender=model_class, 
                      dispatch_uid="att_post_save_"+model_class.__name__)

(Disclaimer: I have not tested the above)

Answer 3

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)

Answer 4

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).

Answer 5

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.

Answer 6

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