[1.7.x] Fixed #22998 -- Updated the fast_delete logic for GFKs

Backport of 6e2b82fdf6 from master

django/db/models/deletion.py

@@ -138,9 +138,9 @@ class Collector(object):
                 include_hidden=True, include_proxy_eq=True):
             if related.field.rel.on_delete is not DO_NOTHING:
                 return False
-        # GFK deletes
-        for relation in opts.many_to_many:
-            if not relation.rel.through:
+        for field in model._meta.virtual_fields:
+            if hasattr(field, 'bulk_related_objects'):
+                # It's something like generic foreign key.
                 return False
         return True
 

tests/generic_relations_regress/models.py

@@ -4,6 +4,7 @@ from django.contrib.contenttypes.fields import (
 from django.contrib.contenttypes.models import ContentType
 from django.db import models
 from django.utils.encoding import python_2_unicode_compatible
+from django.db.models.deletion import ProtectedError
 
 
 __all__ = ('Link', 'Place', 'Restaurant', 'Person', 'Address',
@@ -192,3 +193,26 @@ class D(models.Model):
 
     class Meta:
         ordering = ('id',)
+
+
+# Ticket #22998
+
+class Node(models.Model):
+    content_type = models.ForeignKey(ContentType)
+    object_id = models.PositiveIntegerField()
+    content = GenericForeignKey('content_type', 'object_id')
+
+
+class Content(models.Model):
+    nodes = GenericRelation(Node)
+    related_obj = models.ForeignKey('Related', on_delete=models.CASCADE)
+
+
+class Related(models.Model):
+    pass
+
+
+def prevent_deletes(sender, instance, **kwargs):
+    raise ProtectedError("Not allowed to delete.", [instance])
+
+models.signals.pre_delete.connect(prevent_deletes, sender=Node)

tests/generic_relations_regress/tests.py

@@ -2,11 +2,14 @@ from django.db.models import Q, Sum
 from django.db.utils import IntegrityError
 from django.test import TestCase, skipIfDBFeature
 from django.forms.models import modelform_factory
+from django.db.models.deletion import ProtectedError
 
 from .models import (
     Address, Place, Restaurant, Link, CharLink, TextLink,
     Person, Contact, Note, Organization, OddRelation1, OddRelation2, Company,
-    Developer, Team, Guild, Tag, Board, HasLinkThing, A, B, C, D)
+    Developer, Team, Guild, Tag, Board, HasLinkThing, A, B, C, D,
+    Related, Content, Node,
+)
 
 
 class GenericRelationTests(TestCase):
@@ -247,3 +250,13 @@ class GenericRelationTests(TestCase):
         form.save()
         links = HasLinkThing._meta.get_field_by_name('links')[0]
         self.assertEqual(links.save_form_data_calls, 1)
+
+    def test_ticket_22998(self):
+        related = Related.objects.create()
+        content = Content.objects.create(related_obj=related)
+        node = Node.objects.create(content=content)
+
+        # deleting the Related cascades to the Content cascades to the Node,
+        # where the pre_delete signal should fire and prevent deletion.
+        with self.assertRaises(ProtectedError):
+            related.delete()