#34401: Inconsistent behavior for refresh_from_db() with GenericForeignKey
------------------------------------------------+------------------------
Reporter: François Dupayrat | Owner: nobody
Type: Bug | Status: new
Component: contrib.contenttypes | Version: 4.1
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Since Django 4, `refresh_from_db`exhibit an inconsistent behavior for
`GenericForeignKey` fields: sometime, the object field is properly
refreshed, and sometime it isn't. That issue has been introduced with
Django 4 and I've traced it back to this ticket:
https://code.djangoproject.com/ticket/33008
and in particular those 2 lines in `
django/contrib/contenttypes/fields.py`:
```
if rel_obj is None and self.is_cached(instance):
return rel_obj
```
With those 2 lines removed, the bug doesn't reproduce. I'm unsure if this
is an acceptable solution for Django though.
Here is a minimal example to reproduce this bug.
models.py:
```
from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType
from django.db import models
class FirstType(models.Model):
pass
class SecondType(models.Model):
pass
class FirstOrSecond(models.Model):
association_type = models.ForeignKey(ContentType, blank=True,
null=True, on_delete=models.SET_NULL, limit_choices_to=(
models.Q(app_label='generic_foreign_key_type',
model='firsttype') |
models.Q(app_label='generic_foreign_key_type',
model='secondtype')
))
associated_to_id = models.PositiveIntegerField(blank=True, null=True,
db_index=True)
associated_to = GenericForeignKey('association_type',
'associated_to_id')
def change_association_to(obj_id, target):
matching_obj = FirstOrSecond.objects.filter(id=obj_id).first()
if matching_obj:
matching_obj.associated_to = target
matching_obj.save()
```
tests.py:
```
from django.test import TestCase
from .models import *
class GenericForeignKeysTests(TestCase):
def setUp(self):
self.first_or_second = FirstOrSecond.objects.create()
self.first = FirstType.objects.create()
self.second = SecondType.objects.create()
def test_refresh_from_db(self):
change_association_to(self.first_or_second.id, self.first)
self.first_or_second.refresh_from_db()
self.assertEqual(self.first_or_second.associated_to, self.first)
change_association_to(self.first_or_second.id, None)
self.first_or_second.refresh_from_db()
self.assertEqual(self.first_or_second.associated_to, None)
change_association_to(self.first_or_second.id, self.first)
self.first_or_second.refresh_from_db()
self.assertEqual(self.first_or_second.associated_to, self.first)
```
The last assertion fails because of this bug.
--
Ticket URL: <https://code.djangoproject.com/ticket/34401>
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