Make system.xblock_model_data more explicit to fix the problem where error descriptors can't get any data because the schema and the data don't match.

cms/djangoapps/contentstore/views.py

@@ -454,9 +454,9 @@ def preview_module_system(request, preview_id, descriptor):
     descriptor: An XModuleDescriptor
     """
 
-    def preview_model_data(model_data):
+    def preview_model_data(descriptor):
         return DbModel(
-            SessionKeyValueStore(request, model_data),
+            SessionKeyValueStore(request, descriptor._model_data),
             descriptor.module_class,
             preview_id,
             MongoUsage(preview_id, descriptor.location.url()),

common/lib/xmodule/xmodule/tests/__init__.py

@@ -31,7 +31,7 @@ i4xs = ModuleSystem(
     xqueue={'interface':None, 'callback_url':'/', 'default_queuename': 'testqueue', 'waittime': 10},
     node_path=os.environ.get("NODE_PATH", "/usr/local/lib/node_modules"),
     anonymous_student_id = 'student',
-    xblock_model_data = lambda x: x,
+    xblock_model_data = lambda descriptor: descriptor._model_data,
 )
 
 

common/lib/xmodule/xmodule/x_module.py

@@ -362,7 +362,7 @@ class XModuleDescriptor(HTMLSnippet, ResourceTemplates, XBlock):
             system,
             self.location,
             self,
-            system.xblock_model_data(self._model_data),
+            system.xblock_model_data(self),
         )
     
     def has_dynamic_children(self):

lms/djangoapps/courseware/module_render.py

@@ -203,9 +203,9 @@ def _get_module(user, request, location, student_module_cache, course_id, positi
         return get_module(user, request, location,
                                        student_module_cache, course_id, position)
 
-    def xblock_model_data(descriptor_model_data):
+    def xblock_model_data(descriptor):
         return DbModel(
-            LmsKeyValueStore(course_id, user, descriptor_model_data, student_module_cache),
+            LmsKeyValueStore(course_id, user, descriptor._model_data, student_module_cache),
             descriptor.module_class,
             user.id,
             LmsUsage(location, location)