Replace use_for_related_fields by Meta.base_manager_name

Deprecated in 1.10, removed in 2.0

More information:
- https://code.djangoproject.com/ticket/14891
- django/django@ed0ff913c6

polymorphic/managers.py

@@ -23,8 +23,6 @@ class PolymorphicManager(models.Manager):
     Usually not explicitly needed, except if a custom manager or
     a custom queryset class is to be used.
     """
-    # Tell Django that related fields also need to use this manager:
-    use_for_related_fields = True
     queryset_class = PolymorphicQuerySet
 
     @classmethod

polymorphic/models.py

@@ -40,9 +40,6 @@ class PolymorphicModel(six.with_metaclass(PolymorphicModelBase, models.Model)):
     # for PolymorphicQuery, True => an overloaded __repr__ with nicer multi-line output is used by PolymorphicQuery
     polymorphic_query_multiline_output = False
 
-    class Meta:
-        abstract = True
-
     # avoid ContentType related field accessor clash (an error emitted by model validation)
     #: The model field that stores the :class:`~django.contrib.contenttypes.models.ContentType` reference to the actual class.
     polymorphic_ctype = models.ForeignKey(
@@ -58,6 +55,10 @@ class PolymorphicModel(six.with_metaclass(PolymorphicModelBase, models.Model)):
     objects = PolymorphicManager()
     base_objects = models.Manager()
 
+    class Meta:
+        abstract = True
+        base_manager_name = "objects"
+
     @classmethod
     def translate_polymorphic_Q_object(cls, q):
         return translate_polymorphic_Q_object(cls, q)