Add display value method

Returns the text representation of the instance. Subclasses can override this method to provide a different display value used for populating the `choices` property.

rest_framework/relations.py

@@ -148,7 +148,7 @@ class RelatedField(Field):
         return OrderedDict([
             (
                 six.text_type(self.to_representation(item)),
-                six.text_type(item)
+                six.text_type(self.display_value(item))
             )
             for item in queryset
         ])
@@ -160,6 +160,9 @@ class RelatedField(Field):
     def iter_options(self):
         return iter_options(self.grouped_choices)
 
+    def display_value(self, instance):
+        return six.text_type(instance)
+
 
 class StringRelatedField(RelatedField):
     """