Don't freeze the current query state when calling .order_by()

This changes order_by() to eliminate its reference to
self._cursor. This meant that any parameters built by QuerySet
that followed an order_by() clause were ignored.

mongoengine/queryset.py

@@ -586,11 +586,13 @@ def _cursor(self):
             if self._where_clause:
                 self._cursor_obj.where(self._where_clause)
 
-            # apply default ordering
             if self._ordering:
+                # Apply query ordering
                 self._cursor_obj.sort(self._ordering)
             elif self._document._meta['ordering']:
+                # Otherwise, apply the ordering from the document model
                 self.order_by(*self._document._meta['ordering'])
+                self._cursor_obj.sort(self._ordering)
 
             if self._limit is not None:
                 self._cursor_obj.limit(self._limit - (self._skip or 0))
@@ -1274,7 +1276,7 @@ def order_by(self, *keys):
             key_list.append((key, direction))
 
         self._ordering = key_list
-        self._cursor.sort(key_list)
+
         return self
 
     def explain(self, format=False):

tests/test_queryset.py

@@ -1793,6 +1793,22 @@ def test_order_by(self):
         ages = [p.age for p in self.Person.objects.order_by('-name')]
         self.assertEqual(ages, [30, 40, 20])
 
+    def test_order_by_chaining(self):
+        """Ensure that an order_by query chains properly and allows .only()
+        """
+        self.Person(name="User A", age=20).save()
+        self.Person(name="User B", age=40).save()
+        self.Person(name="User C", age=30).save()
+
+        only_age = self.Person.objects.order_by('-age').only('age')
+
+        names = [p.name for p in only_age]
+        ages = [p.age for p in only_age]
+
+        # The .only('age') clause should mean that all names are None
+        self.assertEqual(names, [None, None, None])
+        self.assertEqual(ages, [40, 30, 20])
+
     def test_confirm_order_by_reference_wont_work(self):
         """Ordering by reference is not possible.  Use map / reduce.. or
         denormalise"""