Added tests for m2m queries with custom pk on the end models

It seems this case was fixed somewhere between 1.5.x and 1.6.x. I added
tests as I wasn't able to find any tests for these cases. Refs #21879

tests/queries/tests.py

@@ -27,7 +27,7 @@ from .models import (
     BaseA, FK1, Identifier, Program, Channel, Page, Paragraph, Chapter, Book,
     MyObject, Order, OrderItem, SharedConnection, Task, Staff, StaffUser,
     CategoryRelationship, Ticket21203Parent, Ticket21203Child, Person,
-    Company, Employment)
+    Company, Employment, CustomPk, CustomPkTag)
 
 
 class BaseQuerysetTest(TestCase):
@@ -3243,3 +3243,16 @@ class ForeignKeyToBaseExcludeTests(TestCase):
             SpecialCategory.objects.filter(categoryitem__id=c1.pk),
             [sc1], lambda x: x
         )
+
+
+class ReverseM2MCustomPkTests(TestCase):
+    def test_ticket_21879(self):
+        cpt1 = CustomPkTag.objects.create(id='cpt1', tag='cpt1')
+        cp1 = CustomPk.objects.create(name='cp1', extra='extra')
+        cp1.custompktag_set.add(cpt1)
+        self.assertQuerysetEqual(
+            CustomPk.objects.filter(custompktag=cpt1), [cp1],
+            lambda x: x)
+        self.assertQuerysetEqual(
+            CustomPkTag.objects.filter(custom_pk=cp1), [cpt1],
+            lambda x: x)