post_delete signal called for both parent and child models, causing orders to be incorrect

[kylepollina]

Issue

If I have a parent and child model defined, ordered with respect to some other model:

class Foobar(models.Model):
    name = models.CharField(max_length=100)

class Parent(OrderedModel):
    name = models.CharField(max_length=100)
    foobar = models.ForeignKey(Foobar, on_delete=models.CASCADE)
    
    order_with_respect_to = 'foobar'
    
class Child(Parent):
    age = models.IntegerField()

When calling delete() on one of the child models, the related parent model is also deleted, since Django stores a pointer from the child model to the parent model in the database. This causes the post_delete signal to be run for both the child model and the parent model. Each time the post_delete signal is called, the orders are shifted to accommodate.

import pytest
@pytest.mark.django_db
def test_parent_child_order():
    foobar = Foobar.objects.create(name="foobar")
    child1 = Child.objects.create(name="child1", foobar=foobar, age=1)
    child2 = Child.objects.create(name="child2", foobar=foobar, age=2)
    child3 = Child.objects.create(name="child3", foobar=foobar, age=3)
    child4 = Child.objects.create(name="child4", foobar=foobar, age=4)
    
    # This is the order of the children at the start
    assert child1.order == 0
    assert child2.order == 1
    assert child3.order == 2
    assert child4.order == 3
    
    # Delete the first child
    # This causes the parent to be deleted as well
    child1.delete()
    
    # Refresh the db
    child2.refresh_from_db()
    child3.refresh_from_db()
    child4.refresh_from_db()

    # The order of the children should be updated
    # The expected order
    assert child2.order == 0
    assert child3.order == 1
    assert child4.order == 2

This test produces this outcome:

>       assert child3.order == 1
E       assert 0 == 1
E        +  where 0 = <Child: Child object (3)>.order
 AssertionError

You would expect child3.order to be 1, but since the post_delete is called twice, it gets shifted twice. In reality, child2.order is 0, child3.order is 0, and child4.order is 1

Expected result

I would expect that the parent models do not have the post_delete signal called on them. As that is unexpected behavior to have the orders shifted twice.

Workaround

I found that I could disconnect the signal from the parent model, in my apps.py file:

from django.apps import AppConfig

from django.db.models.signals import post_delete


class MyAppConfig(AppConfig):
    ...
    def ready(self):
        from myapp.models import Parent
        res = post_delete.disconnect(sender=Parent, dispatch_uid=Parent.__name__)
        if res == False:
            raise AssertionError("Failed to disconnect post_delete signal from Parent model. Make sure that `ordered_model` is loaded before `myapp` in the settings.py file.")

This only works if my app is set after the ordered_model app in the INSTALLED_APPS list in settings.py

INSTALLED_APPS = [
    ...,
    "ordered_model",
    "myapp",
    ...
]