Add __str__ method to PKOnlyObject (#4423)

rest_framework/relations.py

@@ -10,7 +10,7 @@ from django.core.urlresolvers import (
 from django.db.models import Manager
 from django.db.models.query import QuerySet
 from django.utils import six
-from django.utils.encoding import smart_text
+from django.utils.encoding import python_2_unicode_compatible, smart_text
 from django.utils.six.moves.urllib import parse as urlparse
 from django.utils.translation import ugettext_lazy as _
 
@@ -47,6 +47,7 @@ class Hyperlink(six.text_type):
     is_hyperlink = True
 
 
+@python_2_unicode_compatible
 class PKOnlyObject(object):
     """
     This is a mock object, used for when we only need the pk of the object
@@ -56,6 +57,9 @@ class PKOnlyObject(object):
     def __init__(self, pk):
         self.pk = pk
 
+    def __str__(self):
+        return "%s" % self.pk
+
 
 # We assume that 'validators' are intended for the child serializer,
 # rather than the parent serializer.