Add a failing test to demonstrate a bug

polymorphic/tests.py

@@ -167,6 +167,22 @@ class ModelUnderRelChild(PolymorphicModel):
     _private2 = models.CharField(max_length=10)
 
 
+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')
+
+
 class MyManagerQuerySet(PolymorphicQuerySet):
 
     def my_queryset_foo(self):
@@ -1325,6 +1341,44 @@ def func():
         # Ensure no queries are made using the default database.
         self.assertNumQueries(0, func)
 
+    def test_unknown_issue(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).prefetch_related('user_profiles').distinct()
+
+        print(my_teams[0].user_profiles.all())
+        print(my_teams[1].user_profiles.all())
+        self.assertEqual(len(my_teams[0].user_profiles.all()), 3)
+        self.assertEqual(len(my_teams[1].user_profiles.all()), 1)
+
+        print(my_teams[0].user_profiles.all())
+        print(my_teams[1].user_profiles.all())
+        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
+
+        print(my_teams[0].user_profiles.all())
+        print(my_teams[1].user_profiles.all())
+        # This time, test fails.
+        # 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)
+
 
 def qrepr(data):
     """