Add field iterator which includes namespaced fields

And fix get_explicitly_set to use it and to get the json not native
field values.

cms/djangoapps/contentstore/views/item.py

@@ -58,14 +58,12 @@ def save_item(request):
         # 'apply' the submitted metadata, so we don't end up deleting system metadata
         existing_item = modulestore().get_item(item_location)
         for metadata_key in request.POST.get('nullout', []):
-            # [dhm] see comment on _get_xblock_field
             _get_xblock_field(existing_item, metadata_key).write_to(existing_item, None)
 
         # update existing metadata with submitted metadata (which can be partial)
         # IMPORTANT NOTE: if the client passed 'null' (None) for a piece of metadata that means 'remove it'. If
         # the intent is to make it None, use the nullout field
         for metadata_key, value in request.POST.get('metadata', {}).items():
-            # [dhm] see comment on _get_xblock_field
             field = _get_xblock_field(existing_item, metadata_key)
 
             if value is None:
@@ -82,32 +80,15 @@ def save_item(request):
     return JsonResponse()
 
 
-# [DHM] A hack until we implement a permanent soln. Proposed perm solution is to make namespace fields also top level
-# fields in xblocks rather than requiring dereference through namespace but we'll need to consider whether there are
-# plausible use cases for distinct fields w/ same name in different namespaces on the same blocks.
-# The idea is that consumers of the xblock, and particularly the web client, shouldn't know about our internal
-# representation (namespaces as means of decorating all modules).
-# Given top-level access, the calls can simply be setattr(existing_item, field, value) ...
-# Really, this method should be elsewhere (e.g., xblock). We also need methods for has_value (v is_default)...
 def _get_xblock_field(xblock, field_name):
     """
     A temporary function to get the xblock field either from the xblock or one of its namespaces by name.
     :param xblock:
     :param field_name:
     """
-    def find_field(fields):
-        for field in fields:
-            if field.name == field_name:
-                return field
-
-    found = find_field(xblock.fields)
-    if found:
-        return found
-    for namespace in xblock.namespaces:
-        found = find_field(getattr(xblock, namespace).fields)
-        if found:
-            return found
-
+    for field in xblock.iterfields():
+        if field.name == field_name:
+            return field
 
 @login_required
 @expect_json