Fix PK not cleared after fast delete on models with no dependencies

Previously, deleting a model instance with no dependencies did not reset its primary key to None. This ensures that after a .delete() call, the instance's PK is cleared, matching expected behavior.

Adds a test case for models using the fast delete path.

django/db/models/deletion.py

@@ -277,6 +277,7 @@ class Collector:
             if self.can_fast_delete(instance):
                 with transaction.mark_for_rollback_on_error():
                     count = sql.DeleteQuery(model).delete_batch([instance.pk], self.using)
+                setattr(instance, model._meta.pk.attname, None)
                 return count, {model._meta.label: count}
 
         with transaction.atomic(using=self.using, savepoint=False):

tests/delete/models.py

@@ -126,3 +126,8 @@ class Base(models.Model):
 
 class RelToBase(models.Model):
     base = models.ForeignKey(Base, models.DO_NOTHING)
+
+
+class Origin(models.Model):
+    """Model with no reverse relations and no dependencies."""
+    name = models.CharField(max_length=100)

tests/delete/tests.py

@@ -6,7 +6,7 @@ from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 
 from .models import (
     MR, A, Avatar, Base, Child, HiddenUser, HiddenUserProfile, M, M2MFrom,
-    M2MTo, MRNull, Parent, R, RChild, S, T, User, create_a, get_default_r,
+    M2MTo, MRNull, Origin, Parent, R, RChild, S, T, User, create_a, get_default_r,
 )
 
 
@@ -522,3 +522,15 @@ class FastDeleteTests(TestCase):
                 User.objects.filter(avatar__desc='missing').delete(),
                 (0, {'delete.User': 0})
             )
+
+    def test_fast_delete_instance_set_pk_to_none(self):
+        """
+        Test that deleting an instance with no dependencies clears its PK.
+        """
+        # Origin model has no dependencies, should use fast delete path
+        origin = Origin.objects.create(name='test')
+        origin_pk = origin.pk
+        self.assertIsNotNone(origin_pk)
+        origin.delete()
+        # After delete, pk should be None
+        self.assertIsNone(origin.pk)