[1.6.x] Fixed #23604 -- Allowed related m2m fields to be references in the admin.

Thanks Simon Charette for review.

Backport of a24cf21722 from master

tests/admin_views/admin.py

@@ -31,7 +31,7 @@ from .models import (Article, Chapter, Account, Media, Child, Parent, Picture,
     AdminOrderedCallable, Report, Color2, UnorderedObject, MainPrepopulated,
     RelatedPrepopulated, UndeletableObject, UserMessenger, Simple, Choice,
     ShortMessage, Telegram, ReferencedByParent, ChildOfReferer, M2MReference,
-    ReferencedByInline, InlineReference, InlineReferer)
+    ReferencedByInline, InlineReference, InlineReferer, Ingredient)
 
 
 def callable_year(dt_value):
@@ -789,6 +789,7 @@ site.register(Color2, CustomTemplateFilterColorAdmin)
 site.register(Simple, AttributeErrorRaisingAdmin)
 site.register(UserMessenger, MessageTestingAdmin)
 site.register(Choice, ChoiceList)
+site.register(Ingredient)
 
 # Register core models we need in our tests
 from django.contrib.auth.models import User, Group

tests/admin_views/models.py

@@ -716,3 +716,12 @@ class InlineReference(models.Model):
 class InlineReferer(models.Model):
     refs = models.ManyToManyField(InlineReference)
 
+
+# Models for #23604
+class Recipe(models.Model):
+    name = models.CharField(max_length=20)
+
+
+class Ingredient(models.Model):
+    name = models.CharField(max_length=20)
+    recipes = models.ManyToManyField('Recipe', related_name='ingredients')

tests/admin_views/tests.py

@@ -599,6 +599,10 @@ class AdminViewBasicTest(AdminViewBasicTestCase):
         response = self.client.get("/test_admin/admin/admin_views/m2mreference/", {TO_FIELD_VAR: 'id'})
         self.assertEqual(response.status_code, 200)
 
+        # #23604 - Specifying the pk of this model should be allowed when this model defines a m2m relationship
+        response = self.client.get("/test_admin/admin/admin_views/ingredient/", {TO_FIELD_VAR: 'id'})
+        self.assertEqual(response.status_code, 200)
+
         # #23329 - Specifying a field that is not refered by any other model directly registered
         # to this admin site but registered through inheritance should be allowed.
         response = self.client.get("/test_admin/admin/admin_views/referencedbyparent/", {TO_FIELD_VAR: 'id'})