Add a regression test for polymorphic iteration

src/polymorphic/tests/migrations/0001_initial.py

@@ -1,4 +1,4 @@
-# Generated by Django 4.2.26 on 2025-12-01 11:40
+# Generated by Django 4.2.26 on 2025-12-02 16:47
 
 from django.db import migrations, models
 import django.db.models.deletion
@@ -197,6 +197,17 @@ class Migration(migrations.Migration):
                 'base_manager_name': 'objects',
             },
         ),
+        migrations.CreateModel(
+            name='Participant',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('polymorphic_ctype', models.ForeignKey(editable=False, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='polymorphic_%(app_label)s.%(class)s_set+', to='contenttypes.contenttype')),
+            ],
+            options={
+                'abstract': False,
+                'base_manager_name': 'objects',
+            },
+        ),
         migrations.CreateModel(
             name='PlainA',
             fields=[
@@ -591,6 +602,18 @@ class Migration(migrations.Migration):
             },
             bases=('tests.modelshow1_plain',),
         ),
+        migrations.CreateModel(
+            name='UserProfile',
+            fields=[
+                ('participant_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='tests.participant')),
+                ('name', models.CharField(max_length=100)),
+            ],
+            options={
+                'abstract': False,
+                'base_manager_name': 'objects',
+            },
+            bases=('tests.participant',),
+        ),
         migrations.CreateModel(
             name='UUIDArtProject',
             fields=[
@@ -1050,6 +1073,14 @@ class Migration(migrations.Migration):
             ],
             bases=('tests.uuidplainb',),
         ),
+        migrations.CreateModel(
+            name='Team',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('team_name', models.CharField(max_length=100)),
+                ('user_profiles', models.ManyToManyField(related_name='user_teams', to='tests.userprofile')),
+            ],
+        ),
         migrations.CreateModel(
             name='InlineModelB',
             fields=[

src/polymorphic/tests/models.py

@@ -519,3 +519,19 @@ class SubclassSelectorProxyConcreteModel(SubclassSelectorProxyModel):
 
 class NonPolymorphicParent(PolymorphicModel, Group):
     test = models.CharField(max_length=30, default="test_non_polymorphic_parent")
+
+
+class Participant(PolymorphicModel):
+    pass
+
+
+class UserProfile(Participant):
+    name = models.CharField(max_length=100)
+
+    def __str__(self):
+        return self.name
+
+
+class Team(models.Model):
+    team_name = models.CharField(max_length=100)
+    user_profiles = models.ManyToManyField(UserProfile, related_name="user_teams")

src/polymorphic/tests/test_regression.py

@@ -1,6 +1,6 @@
 from django.test import TestCase
 
-from polymorphic.tests.models import Bottom, Middle, Top
+from polymorphic.tests.models import Bottom, Middle, Top, Team, UserProfile
 
 
 class RegressionTests(TestCase):
@@ -34,3 +34,41 @@ def test_for_query_result_incomplete_with_inheritance(self):
             [repr(r) for r in expected_queryset],
             transform=repr,
         )
+
+    def test_pr_254(self):
+        user_a = UserProfile.objects.create(name="a")
+        user_b = UserProfile.objects.create(name="b")
+        user_c = UserProfile.objects.create(name="c")
+
+        team1 = Team.objects.create(team_name="team1")
+        team1.user_profiles.add(user_a, user_b, user_c)
+        team1.save()
+
+        team2 = Team.objects.create(team_name="team2")
+        team2.user_profiles.add(user_c)
+        team2.save()
+
+        # without prefetch_related, the test passes
+        my_teams = (
+            Team.objects.filter(user_profiles=user_c)
+            .order_by("team_name")
+            .prefetch_related("user_profiles")
+            .distinct()
+        )
+
+        self.assertEqual(len(my_teams[0].user_profiles.all()), 3)
+
+        self.assertEqual(len(my_teams[1].user_profiles.all()), 1)
+
+        self.assertEqual(len(my_teams[0].user_profiles.all()), 3)
+        self.assertEqual(len(my_teams[1].user_profiles.all()), 1)
+
+        # without this "for" loop, the test passes
+        for _ in my_teams:
+            pass
+
+        # This time, test fails.  PR 254 claim
+        # with sqlite:      4 != 3
+        # with postgresql:  2 != 3
+        self.assertEqual(len(my_teams[0].user_profiles.all()), 3)
+        self.assertEqual(len(my_teams[1].user_profiles.all()), 1)