Merge pull request django-json-api#161 from django-json-api/bugfix/obscure-relation-error

When a `pk` was used as a relationship value a json error was thrown

Author: jsenecal

rest_framework_json_api/relations.py

@@ -109,7 +109,11 @@ def get_links(self, obj=None, lookup_field='pk'):
 
     def to_internal_value(self, data):
         if isinstance(data, six.text_type):
-            data = json.loads(data)
+            try:
+                data = json.loads(data)
+            except ValueError:
+                # show a useful error if they send a `pk` instead of resource object
+                self.fail('incorrect_type', data_type=type(data).__name__)
         if not isinstance(data, dict):
             self.fail('incorrect_type', data_type=type(data).__name__)
         expected_relation_type = get_resource_type_from_queryset(self.queryset)