MixedModulestore wraps most getters, update_item, delete_item

with code to translate between addressing schemes based on app
and persistence layer addressing scheme specification.

STUD-1206

lms/djangoapps/instructor_task/tests/test_integration.py

@@ -288,7 +288,11 @@ class TestRescoringTask(TestIntegrationTask):
             """ % ('!=' if redefine else '=='))
         problem_xml = factory.build_xml(script=script, cfn="check_func", expect="42", num_responses=1)
         if redefine:
-            self.module_store.update_item(InstructorTaskModuleTestCase.problem_location(problem_url_name), problem_xml)
+            descriptor = self.module_store.get_instance(
+                self.course.id, InstructorTaskModuleTestCase.problem_location(problem_url_name)
+            )
+            descriptor.data = problem_xml
+            self.module_store.update_item(descriptor, 'define_randomized_custom_response_problem')
         else:
             # Use "per-student" rerandomization so that check-problem can be called more than once.
             # Using "always" means we cannot check a problem twice, but we want to call once to get the