Add __len__ to FieldDataCache

lms/djangoapps/courseware/model_data.py

@@ -951,3 +951,6 @@ class FieldDataCache(object):
             return None
 
         return self.cache[key.scope].last_modified(key)
+
+    def __len__(self):
+        return sum(len(cache) for cache in self.cache.values())

lms/djangoapps/courseware/tests/test_model_data.py

@@ -242,7 +242,7 @@ class TestMissingStudentModule(TestCase):
 
     def test_set_field_in_missing_student_module(self):
         "Test that setting a field in a missing StudentModule creates the student module"
-        self.assertEquals(0, sum(len(cache) for cache in self.field_data_cache.cache.values()))
+        self.assertEquals(0, len(self.field_data_cache))
         self.assertEquals(0, StudentModule.objects.all().count())
 
         # We are updating a problem, so we write to courseware_studentmodulehistory