Fixed internal usage of deprecated `base_objects`

Reverted the change in f898f80594 that
also replaces the internal `base_objects` with `.non_polymorphic()`.
That also changed which querysets was used. Use a clean queryset
instead that has no select-related/prefetch information, etc.. like
previous versions did.

polymorphic/base.py

@@ -163,8 +163,11 @@ class PolymorphicModelBase(ModelBase):
         warnings.warn(
             "Using PolymorphicModel.base_objects is deprecated.\n"
             "Use {0}.objects.non_polymorphic() instead.".format(self.__class__.__name__),
-            DeprecationWarning)
+            DeprecationWarning, stacklevel=2)
+        return self._base_objects
 
+    @property
+    def _base_objects(self):
         # Create a manager so the API works as expected. Just don't register it
         # anymore in the Model Meta, so it doesn't substitute our polymorphic
         # manager as default manager for the third level of inheritance when
@@ -188,7 +191,7 @@ class PolymorphicModelBase(ModelBase):
             # for all supported Django versions.
             frm = inspect.stack()[1]  # frm[1] is caller file name, frm[3] is caller function name
             if DUMPDATA_COMMAND in frm[1]:
-                return self.base_objects
+                return self._base_objects
 
         manager = super(PolymorphicModelBase, self)._default_manager
         if not isinstance(manager, PolymorphicManager):

polymorphic/models.py

@@ -172,7 +172,7 @@ class PolymorphicModel(six.with_metaclass(PolymorphicModelBase, models.Model)):
 
         def create_accessor_function_for_model(model, accessor_name):
             def accessor_function(self):
-                attr = model.base_objects.get(pk=self.pk)
+                attr = model._base_objects.get(pk=self.pk)
                 return attr
             return accessor_function
 

polymorphic/query.py

@@ -379,7 +379,7 @@ class PolymorphicQuerySet(QuerySet):
         # Then we copy the extra() select fields from the base objects to the real objects.
         # TODO: defer(), only(): support for these would be around here
         for real_concrete_class, idlist in idlist_per_model.items():
-            real_objects = real_concrete_class.objects.non_polymorphic().using(self.db).filter(**{
+            real_objects = real_concrete_class._base_objects.db_manager(self.db).filter(**{
                 ('%s__in' % pk_name): idlist,
             })
             real_objects.query.select_related = self.query.select_related  # copy select related configuration to new qs