fix url field representation

when lookup_field is a foreignkey, lookup_value should get from the
referenced object's lookup_field(which is lookup_url_kwarg here)
when lookup_value is None ,'' or not a string , HyperlinkedRelatedField
should return None instead of http://foo/bar/None['', or just exception] for correct url
reversing

rest_framework/relations.py

@@ -201,6 +201,22 @@ class HyperlinkedRelatedField(RelatedField):
             return None
 
         lookup_value = getattr(obj, self.lookup_field)
+
+        import numbers
+        from django.utils import six
+
+        if lookup_value is not None and not isinstance(lookup_value, six.string_types) and not isinstance(lookup_value,
+                                                                                                          numbers.Number):
+
+            try:
+                lookup_value = getattr(lookup_value, self.lookup_url_kwarg)
+            except Exception:
+                lookup_value = getattr(lookup_value, 'pk')
+
+        if lookup_value is None or (not isinstance(lookup_value, six.string_types) and not isinstance(lookup_value,
+                                                                                                      numbers.Number)) or '' == lookup_value:
+            return None
+
         kwargs = {self.lookup_url_kwarg: lookup_value}
         return self.reverse(view_name, kwargs=kwargs, request=request, format=format)