Unable to Override GenericForeignKey in Inherited Abstract Class

[Gagan Deep]

Attempting to override a GenericForeignKey field in an inherited abstract model does not work as expected. According to the ​Django documentation, it should be possible to hide a field inherited from an abstract model by setting it to None.

However, this does not seem to work for GenericForeignKey.

I have created a ​Django project for quick testing.

Steps to replicate:

1. Define an abstract model with a GenericForeignKey field.
2. Create another abstract model that inherits from the first and attempt to override the GenericForeignKey field by setting it to None.
3. Define a concrete model that inherits from the second abstract model.

Code Example

from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType
from django.db import models


class AbstractBaseModel(models.Model):
    content_type = models.ForeignKey(
        ContentType, on_delete=models.SET_NULL, null=True, blank=True
    )
    object_id = models.PositiveIntegerField(null=True, blank=True)
    related_object = GenericForeignKey("content_type", "object_id")
    description = models.TextField(null=True, blank=True)

    class Meta:
        abstract = True


class AbstractDerivedModel(AbstractBaseModel):
    related_object = None  # Override GenericForeignKey
    description = None

    class Meta:
        abstract = True


class ConcreteEntity(AbstractDerivedModel):
    name = models.CharField(max_length=255)

    class Meta:
        abstract = False

Expected Behavior:

The related_object field in AbstractDerivedModel should override the GenericForeignKey in AbstractBaseModel, making it effectively absent in ConcreteEntity.

Actual Behavior:

Django still considers the GenericForeignKey field from AbstractBaseModel when defining ConcreteEntity, leading to unexpected behavior.

Additional Notes:

• Overriding standard model fields with None works as expected, but GenericForeignKey does not follow the same behavior (description field in the above example).
• There is no explicit mention in the documentation that GenericForeignKey is exempt from the field-hiding mechanism. I checked the following pages:
  • ​https://docs.djangoproject.com/en/5.1/ref/contrib/contenttypes/
  • ​https://docs.djangoproject.com/en/5.2/topics/db/models/#field-name-hiding-is-not-permitted

[Sarah Boyce]

Thank you!
Using the models as defined above, I was able to replicate

tests/generic_relations/tests.py

@@ -1 +1 @@
 from django.contrib.contenttypes.models import ContentType
 from django.contrib.contenttypes.prefetch import GenericPrefetch
-from django.core.exceptions import FieldError
+from django.core.exceptions import FieldError, FieldDoesNotExist
 from django.db.models import Q, prefetch_related_objects
 from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
 
@@ -9 +9 @@
     Animal,
     Carrot,
     Comparison,
+    ConcreteEntity,
     ConcreteRelatedModel,
     ForConcreteModelModel,
     ForProxyModelModel,
@@ -780 +781 @@
                 self.platypus.latin_name,
             )
 
+    def test_overwriting_generic_fk(self):
+        with self.assertRaises(FieldDoesNotExist):
+            ConcreteEntity._meta.get_field('description')
+        with self.assertRaises(FieldDoesNotExist):
+            ConcreteEntity._meta.get_field('related_object')
+
 
 class ProxyRelatedModelTest(TestCase):

[Gagan Deep]

Hello Sarah! Can I get some pointers how can I approach to solve this issue? I would like to attempt a fix.

[Senthil Kumar]

This is my first time working with the Django source code. After reviewing the codebase, I believe the fix below might address the issue. However, I'm unsure whether this is the expected behavior that requires a documentation update or if it's an actual bug.

File db/models/base.py -- line 348

​https://github.com/django/django/blob/8ebdd37a0b1755842baae3bd34d388156ad4bf53/django/db/models/base.py#L348

if field.column is not None:  # the exact fix
        field = copy.deepcopy(field)
        if not base._meta.abstract:
               field.mti_inherited = True
               new_class.add_to_class(field.name, field)

Apologies, but I'm unsure how to present this from the codebase. It would be helpful if someone could guide me on how to share the diff, similar to the comments above. Thanks!

[Arne]

The PR looks fine to me. Just waiting for some minor, optional adjustments before _ready for checkin_.

[Ahmed Nassar]

Can anyone review? Looking forward to merging this PR.

[Sarah Boyce <42296566+sarahboyce@…>]

In 84e91262:

Fixed #36295, Refs #24305 -- Allowed overriding GenericForeignKey fields on abstract models.