Made model fields comparable to other objects

Fixed #17851 -- Added __lt__ and @total_ordering to models.Field,
made sure these work correctly on other objects than Field, too.

tests/modeltests/basic/tests.py

@@ -3,7 +3,7 @@ from __future__ import absolute_import
 from datetime import datetime
 
 from django.core.exceptions import ObjectDoesNotExist
-from django.db.models.fields import FieldDoesNotExist
+from django.db.models.fields import Field, FieldDoesNotExist
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils.translation import ugettext_lazy
 
@@ -520,6 +520,22 @@ class ModelTest(TestCase):
         s = set([a10, a11, a12])
         self.assertTrue(Article.objects.get(headline='Article 11') in s)
 
+    def test_field_ordering(self):
+        """
+        Field instances have a `__lt__` comparison function to define an
+        ordering based on their creation. Prior to #17851 this ordering
+        comparison relied on the now unsupported `__cmp__` and was assuming
+        compared objects were both Field instances raising `AttributeError`
+        when it should have returned `NotImplemented`.
+        """
+        f1 = Field()
+        f2 = Field(auto_created=True)
+        f3 = Field()
+        self.assertTrue(f2 < f1)
+        self.assertTrue(f3 > f1)
+        self.assertFalse(f1 == None)
+        self.assertFalse(f2 in (None, 1, ''))
+
     def test_extra_method_select_argument_with_dashes_and_values(self):
         # The 'select' argument to extra() supports names with dashes in
         # them, as long as you use values().