Fix unwanted `___` handling when a fieldname starts with an underscore.

docs/changelog.rst

@@ -5,6 +5,8 @@ Version 0.5.2 (unreleased)
 --------------------------
 
 * Fix Grappelli_ breadcrumb support in the views.
+* Fix unwanted ``___`` handling in the ORM when a field name starts with an underscore;
+  this detects you meant ``relatedfield__ _underscorefield`` instead of ``ClassName___field``.
 
 
 Version 0.5.1 (2013-07-05)

polymorphic/query_translate.py

@@ -6,7 +6,8 @@ from __future__ import absolute_import
 
 from django.db import models
 from django.contrib.contenttypes.models import ContentType
-from django.db.models import Q
+from django.db.models import Q, FieldDoesNotExist
+from django.db.models.related import RelatedObject
 
 from functools import reduce
 
@@ -146,9 +147,21 @@ def translate_polymorphic_field_path(queryset_model, field_path):
             raise AssertionError(e)
 
     else:
-        # the user has only given us the class name via __
+        # the user has only given us the class name via ___
         # => select the model from the sub models of the queryset base model
 
+        # Test whether it's actually a regular relation__ _fieldname (the field starting with an _)
+        # so no tripple ClassName___field was intended.
+        try:
+            # rel = (field_object, model, direct, m2m)
+            field = queryset_model._meta.get_field_by_name(classname)[0]
+            if isinstance(field, RelatedObject):
+                # Can also test whether the field exists in the related object to avoid ambiguity between
+                # class names and field names, but that never happens when your class names are in CamelCase.
+                return field_path  # No exception raised, field does exist.
+        except FieldDoesNotExist:
+            pass
+
         # function to collect all sub-models, this should be optimized (cached)
         def add_all_sub_models(model, result):
             if issubclass(model, models.Model) and model != models.Model: