Change the central cache to store at two levels: first by scope, then by cache key

lms/djangoapps/courseware/tests/test_model_data.py

@@ -230,7 +230,7 @@ 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, len(self.field_data_cache.cache))
+        self.assertEquals(0, sum(len(cache) for cache in self.field_data_cache.cache.values()))
         self.assertEquals(0, StudentModule.objects.all().count())
 
         # We are updating a problem, so we write to courseware_studentmodulehistory
@@ -238,7 +238,7 @@ class TestMissingStudentModule(TestCase):
         with self.assertNumQueries(6):
             self.kvs.set(user_state_key('a_field'), 'a_value')
 
-        self.assertEquals(1, len(self.field_data_cache.cache))
+        self.assertEquals(1, sum(len(cache) for cache in self.field_data_cache.cache.values()))
         self.assertEquals(1, StudentModule.objects.all().count())
 
         student_module = StudentModule.objects.all()[0]