[1.11.x] Fixed #27889 -- Fixed incorrect check error if ModelAdmin.ordering refers to 'pk'.

Backport of d82ee32aac from master

django/contrib/admin/checks.py

@@ -468,7 +468,8 @@ class BaseModelAdminChecks(object):
         else:
             if field_name.startswith('-'):
                 field_name = field_name[1:]
-
+            if field_name == 'pk':
+                return []
             try:
                 model._meta.get_field(field_name)
             except FieldDoesNotExist:

tests/modeladmin/test_checks.py

@@ -796,7 +796,7 @@ class OrderingCheckTests(CheckTestCase):
 
     def test_valid_case(self):
         class TestModelAdmin(ModelAdmin):
-            ordering = ('name',)
+            ordering = ('name', 'pk')
 
         self.assertIsValid(TestModelAdmin, ValidationTestModel)