[Django] #35073: New behavior of ForeignKey with on_delete=models.SET (Django 4.2 and 5.0)

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Dec 30, 2023, 12:27:21 PM12/30/23

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With an upgrade from Django 4.1 to 4.2 (but also verified in Django 5.0),
I've noticed a change in behavior with how on_delete=models.SET is
handled.
Given the following models:

{{{
from django.db import models

class Person(models.Model):
name = models.CharField(max_length=32)

def __str__(self):
return self.name

def get_default_person():
return Person.objects.get_or_create(name="ghost")[0]

class Pet(models.Model):
name = models.CharField(max_length=32)
person = models.ForeignKey(Person, related_name="pets",
on_delete=models.SET(get_default_person))

def __str__(self):
return self.name

}}}

I can see what follows in Django 4.2+ (in ./manage.py shell):

{{{
>>> from pets.models import Person, Pet
>>> Person.objects.all()
<QuerySet []>
>>> Pet.objects.all()
<QuerySet []>
>>> Person.objects.create(name="johndoe")
<Person: johndoe>
>>> Person.objects.all()
<QuerySet [<Person: johndoe>]>
>>> Person.objects.all().delete()
(1, {'pets.Person': 1})
>>> Person.objects.all()
<QuerySet [<Person: ghost>]>
}}}

What is strange to me is that the "ghost" Person instance is created upon
deletion of the "johndoe" instance, even if there are no Pets with a
ForeignKey to "johndoe".
Django 4.1 behaves differently (no "ghost" Person is created on deletion
of other Person objects).

Is this an intended change? I couldn't find any documentation of this in
the release notes.

Thanks so much for your help.

Fabio

--
Ticket URL: <https://code.djangoproject.com/ticket/35073>
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Django

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Dec 31, 2023, 4:28:18 AM12/31/23

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#35073: New behavior of ForeignKey with on_delete=models.SET (Django 4.2 and 5.0)
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Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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