Fixed #21431 -- GenRel->FK list_filter regression in admin

Report, analysis and tests from stephenmcd.

django/contrib/admin/utils.py

@@ -389,10 +389,8 @@ class NotRelationField(Exception):
 
 
 def get_model_from_relation(field):
-    if isinstance(field, models.related.RelatedObject):
-        return field.model
-    elif getattr(field, 'rel'):  # or isinstance?
-        return field.rel.to
+    if hasattr(field, 'get_path_info'):
+        return field.get_path_info()[-1].to_opts.model
     else:
         raise NotRelationField
 

tests/admin_views/tests.py

@@ -18,6 +18,7 @@ from django.contrib.admin.helpers import ACTION_CHECKBOX_NAME
 from django.contrib.admin.models import LogEntry, DELETION
 from django.contrib.admin.sites import LOGIN_FORM_KEY
 from django.contrib.admin.utils import quote
+from django.contrib.admin.validation import ModelAdminValidator
 from django.contrib.admin.views.main import IS_POPUP_VAR
 from django.contrib.admin.tests import AdminSeleniumWebDriverTestCase
 from django.contrib.auth import REDIRECT_FIELD_NAME
@@ -4682,3 +4683,20 @@ class InlineAdminViewOnSiteTest(TestCase):
         self.assertContains(response,
                             '"/worker_inline/%s/%s/"' % (worker.surname, worker.name),
                             )
+
+
+class AdminGenericRelationTests(TestCase):
+    def test_generic_relation_fk_list_filter(self):
+        """
+        Validates a model with a generic relation to a model with
+        a foreign key can specify the generic+fk relationship
+        path as a list_filter. See trac #21428.
+        """
+        class GenericFKAdmin(ModelAdmin):
+            list_filter = ('tags__content_type',)
+
+        validator = ModelAdminValidator()
+        try:
+            validator.validate_list_filter(GenericFKAdmin, Plot)
+        except ImproperlyConfigured:
+            self.fail("Couldn't validate a GenericRelation -> FK path in ModelAdmin.list_filter")