Fix "Model" __ "_field" query support

Broken by 24e6b21204 for Django 1.9
upgrade

polymorphic/query_translate.py

@@ -8,14 +8,7 @@ from django.db import models
 from django.contrib.contenttypes.models import ContentType
 from django.db.models import Q, FieldDoesNotExist
 
-try:
-    from django.db.models.related import RelatedObject
-except ImportError:
-    # django.db.models.related.RelatedObject was replaced
-    # by django.db.models.fields.related.ForeignObjectRel in
-    # Django 1.8
-    from django.db.models.fields.related import ForeignObjectRel
-    RelatedObject = ForeignObjectRel
+from django.db.models.fields.related import RelatedField  # Django 1.8
 
 from functools import reduce
 
@@ -163,7 +156,7 @@ def translate_polymorphic_field_path(queryset_model, field_path):
         try:
             # rel = (field_object, model, direct, m2m)
             field = queryset_model._meta.get_field(classname)
-            if isinstance(field, RelatedObject):
+            if isinstance(field, RelatedField):
                 # 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.

polymorphic/tests.py

@@ -153,6 +153,15 @@ class One2OneRelatingModelDerived(One2OneRelatingModel):
     field2 = models.CharField(max_length=10)
 
 
+class ModelUnderRelParent(PolymorphicModel):
+    field1 = models.CharField(max_length=10)
+    _private = models.CharField(max_length=10)
+
+
+class ModelUnderRelChild(PolymorphicModel):
+    parent = models.ForeignKey(ModelUnderRelParent)
+
+
 class MyManagerQuerySet(PolymorphicQuerySet):
 
     def my_queryset_foo(self):
@@ -720,6 +729,14 @@ class PolymorphicTests(TestCase):
         self.assertEqual(repr(objects[0]), '<Model2B: id 2, field1 (CharField), field2 (CharField)>')
         self.assertEqual(repr(objects[1]), '<Model2C: id 3, field1 (CharField), field2 (CharField), field3 (CharField)>')
 
+    def test_polymorphic___filter_field(self):
+        p = ModelUnderRelParent.objects.create(_private=True, field1='AA')
+        ModelUnderRelChild.objects.create(parent=p)
+
+        # The "___" filter should also parse to "parent" -> "_private" as fallback.
+        objects = ModelUnderRelChild.objects.filter(parent___private=True)
+        self.assertEqual(len(objects), 1)
+
     def test_delete(self):
         self.create_model2abcd()