Allow .order_by() to pass expressions unchanged

Fixes: #257

polymorphic/query.py

@@ -172,10 +172,14 @@ class PolymorphicQuerySet(QuerySet):
         additional_args = translate_polymorphic_filter_definitions_in_kwargs(self.model, kwargs, using=self.db)  # filter_field='data'
         return super(PolymorphicQuerySet, self)._filter_or_exclude(negate, *(list(q_objects) + additional_args), **kwargs)
 
-    def order_by(self, *args, **kwargs):
+    def order_by(self, *field_names):
         """translate the field paths in the args, then call vanilla order_by."""
-        new_args = [translate_polymorphic_field_path(self.model, a) for a in args]
-        return super(PolymorphicQuerySet, self).order_by(*new_args, **kwargs)
+        field_names = [
+            translate_polymorphic_field_path(self.model, a)
+            if isinstance(a, six.string_types) else a  # allow expressions to pass unchanged
+            for a in field_names
+        ]
+        return super(PolymorphicQuerySet, self).order_by(*field_names)
 
     def defer(self, *fields):
         """

polymorphic/query_translate.py

@@ -11,6 +11,7 @@ from django.db import models
 from django.contrib.contenttypes.models import ContentType
 from django.db.models import Q, FieldDoesNotExist
 from django.db.utils import DEFAULT_DB_ALIAS
+from django.utils import six
 
 from django.db.models.fields.related import RelatedField
 if django.VERSION < (1, 6):
@@ -145,6 +146,9 @@ def translate_polymorphic_field_path(queryset_model, field_path):
     into modela__modelb__modelc__field3.
     Returns: translated path (unchanged, if no translation needed)
     """
+    if not isinstance(field_path, six.string_types):
+        raise ValueError("Expected field name as string: {0}".format(field_path))
+
     classname, sep, pure_field_path = field_path.partition('___')
     if not sep:
         return field_path