auto generate the discussion id on create. Also add 'discussion_id' to the list of 'system-metadata' so that end users cannot edit it

common/lib/xmodule/xmodule/modulestore/mongo.py

@@ -9,6 +9,7 @@ from fs.osfs import OSFS
 from itertools import repeat
 from path import path
 from datetime import datetime, timedelta
+from uuid import uuid4
 
 from importlib import import_module
 from xmodule.errortracker import null_error_tracker, exc_info_to_str
@@ -496,6 +497,12 @@ class MongoModuleStore(ModuleStoreBase):
         """
         try:
             source_item = self.collection.find_one(location_to_query(source))
+
+            # allow for some programmatically generated substitutions in metadata, e.g. Discussion_id's should be auto-generated
+            for key in source_item['metadata'].keys():
+                if source_item['metadata'][key] == '$$GUID$$':
+                    source_item['metadata'][key] = uuid4().hex
+
             source_item['_id'] = Location(location).dict()
             self.collection.insert(source_item)
             item = self._load_items([source_item])[0]

common/lib/xmodule/xmodule/templates/discussion/default.yaml

@@ -2,7 +2,7 @@
 metadata:
     display_name: Discussion Tag
     for: Topic-Level Student-Visible Label 
-    id: 6002x_group_discussion_by_this 
+    id: $$GUID$$
     discussion_category: Week 1
 data: |
   <discussion />

common/lib/xmodule/xmodule/x_module.py

@@ -340,7 +340,7 @@ class XModuleDescriptor(XModuleFields, HTMLSnippet, ResourceTemplates, XBlock):
 
     # cdodge: this is a list of metadata names which are 'system' metadata
     # and should not be edited by an end-user
-    system_metadata_fields = ['data_dir', 'published_date', 'published_by', 'is_draft']
+    system_metadata_fields = ['data_dir', 'published_date', 'published_by', 'is_draft', 'discussion_id']
 
     # A list of descriptor attributes that must be equal for the descriptors to
     # be equal