Added ability to define backend identification attribute

rest_framework/relations.py

@@ -166,6 +166,7 @@ class HyperlinkedRelatedField(RelatedField):
         self.lookup_field = kwargs.pop('lookup_field', self.lookup_field)
         self.lookup_url_kwarg = kwargs.pop('lookup_url_kwarg', self.lookup_field)
         self.format = kwargs.pop('format', None)
+        self.id_field = kwargs.pop('id_field', 'pk')
 
         # We include this simply for dependency injection in tests.
         # We can't add it as a class attributes or it would expect an
@@ -196,7 +197,7 @@ class HyperlinkedRelatedField(RelatedField):
         attributes are not configured to correctly match the URL conf.
         """
         # Unsaved objects will not yet have a valid URL.
-        if obj.pk is None:
+        if getattr(obj, self.id_field) is None:
             return None
 
         lookup_value = getattr(obj, self.lookup_field)