Add a last_modified method to FieldDataCache

lms/djangoapps/courseware/model_data.py

@@ -282,6 +282,23 @@ class DjangoOrmFieldCache(object):
         """
         return self._cache_key_for_kvs_key(kvs_key) in self._cache
 
+    @contract(kvs_key=DjangoKeyValueStore.Key, returns="datetime|None")
+    def last_modified(self, kvs_key):
+        """
+        Return when the supplied field was changed.
+
+        Arguments:
+            kvs_key (`DjangoKeyValueStore.Key`): The field value to delete
+
+        Returns: datetime if there was a modified date, or None otherwise
+        """
+        field_object = self._cache.get(self._cache_key_for_kvs_key(kvs_key))
+
+        if field_object is None:
+            return None
+        else:
+            return field_object.modified
+
     @contract(kvs_key=DjangoKeyValueStore.Key)
     def _set_field_value(self, field_object, kvs_key, value):
         field_object.value = json.dumps(value)
@@ -933,3 +950,23 @@ class FieldDataCache(object):
         assert user_id == self.user.id
         assert usage_key.course_key == self.course_id
         self.cache[Scope.user_state].set_score(user_id, usage_key, score, max_score)
+
+    @contract(key=DjangoKeyValueStore.Key, returns="datetime|None")
+    def last_modified(self, key):
+        """
+        Return when the supplied field was changed.
+
+        Arguments:
+            key (`DjangoKeyValueStore.Key`): The field value to delete
+
+        Returns: datetime if there was a modified date, or None otherwise
+        """
+        if key.scope.user == UserScope.ONE and not self.user.is_anonymous():
+            # If we're getting user data, we expect that the key matches the
+            # user we were constructed for.
+            assert key.user_id == self.user.id
+
+        if key.scope not in self.cache:
+            return None
+
+        return self.cache[key.scope].last_modified(key)

lms/djangoapps/mobile_api/users/views.py

@@ -149,12 +149,10 @@ class UserCourseStatus(views.APIView):
                 block_scope_id=course.location,
                 field_name=None
             )
-            student_module = field_data_cache.find(key)
-            if student_module:
-                original_store_date = student_module.modified
-                if modification_date < original_store_date:
-                    # old modification date so skip update
-                    return self._get_course_info(request, course)
+            original_store_date = field_data_cache.last_modified(key)
+            if original_store_date is not None and modification_date < original_store_date:
+                # old modification date so skip update
+                return self._get_course_info(request, course)
 
         save_positions_recursively_up(request.user, request, field_data_cache, module)
         return self._get_course_info(request, course)