Fixed #24479 -- Added system check to prevent both ordering and order_wrt.

django/db/models/base.py

@@ -1479,7 +1479,17 @@ class Model(six.with_metaclass(ModelBase)):
     def _check_ordering(cls):
         """ Check "ordering" option -- is it a list of strings and do all fields
         exist? """
-        if not cls._meta.ordering:
+        if cls._meta._ordering_clash:
+            return [
+                checks.Error(
+                    "'ordering' and 'order_with_respect_to' cannot be used together.",
+                    hint=None,
+                    obj=cls,
+                    id='models.E021',
+                ),
+            ]
+
+        if cls._meta.order_with_respect_to or not cls._meta.ordering:
             return []
 
         if not isinstance(cls._meta.ordering, (list, tuple)):
@@ -1502,9 +1512,6 @@ class Model(six.with_metaclass(ModelBase)):
         # Convert "-field" to "field".
         fields = ((f[1:] if f.startswith('-') else f) for f in fields)
 
-        fields = (f for f in fields if
-            f != '_order' or not cls._meta.order_with_respect_to)
-
         # Skip ordering in the format field1__field2 (FIXME: checking
         # this format would be nice, but it's a little fiddly).
         fields = (f for f in fields if '__' not in f)

django/db/models/options.py

@@ -100,6 +100,7 @@ class Options(object):
         self.verbose_name_plural = None
         self.db_table = ''
         self.ordering = []
+        self._ordering_clash = False
         self.unique_together = []
         self.index_together = []
         self.select_on_save = False
@@ -234,6 +235,9 @@ class Options(object):
             if self.verbose_name_plural is None:
                 self.verbose_name_plural = string_concat(self.verbose_name, 's')
 
+            # order_with_respect_and ordering are mutually exclusive.
+            self._ordering_clash = bool(self.ordering and self.order_with_respect_to)
+
             # Any leftover attributes must be invalid.
             if meta_attrs != {}:
                 raise TypeError("'class Meta' got invalid attribute(s): %s" % ','.join(meta_attrs.keys()))

docs/ref/checks.txt

@@ -63,6 +63,8 @@ Models
   ``<M2M field>``. Maximum length is ``<maximum length>`` for database
   ``<alias>``.
 * **models.E020**: The ``<model>.check()`` class method is currently overridden.
+* **models.E021**: ``ordering`` and ``order_with_respect_to`` cannot be used
+  together.
 
 Fields
 ~~~~~~

docs/releases/1.9.txt

@@ -164,6 +164,9 @@ Models
   <django.db.models.Model.delete>` to allow deleting only a child's data in a
   model that uses multi-table inheritance.
 
+* Added a system check to prevent defining both ``Meta.ordering`` and
+  ``order_with_respect_to`` on the same model.
+
 CSRF
 ^^^^
 

tests/invalid_models_tests/test_models.py

@@ -566,6 +566,50 @@ class OtherModelTests(IsolatedModelsTestCase):
         ]
         self.assertEqual(errors, expected)
 
+    def test_just_ordering_no_errors(self):
+        class Model(models.Model):
+            order = models.PositiveIntegerField()
+
+            class Meta:
+                ordering = ['order']
+
+        self.assertEquals(Model.check(), [])
+
+    def test_just_order_with_respect_to_no_errors(self):
+        class Question(models.Model):
+            pass
+
+        class Answer(models.Model):
+            question = models.ForeignKey(Question)
+
+            class Meta:
+                order_with_respect_to = 'question'
+
+        self.assertEquals(Answer.check(), [])
+
+    def test_ordering_with_order_with_respect_to(self):
+        class Question(models.Model):
+            pass
+
+        class Answer(models.Model):
+            question = models.ForeignKey(Question)
+            order = models.IntegerField()
+
+            class Meta:
+                order_with_respect_to = 'question'
+                ordering = ['order']
+
+        errors = Answer.check()
+        expected = [
+            Error(
+                "'ordering' and 'order_with_respect_to' cannot be used together.",
+                hint=None,
+                obj=Answer,
+                id='models.E021',
+            ),
+        ]
+        self.assertEqual(errors, expected)
+
     def test_non_valid(self):
         class RelationModel(models.Model):
             pass