add conversation improvements

add a comment about root account ids being relative to the birth shard
and update some of the references to root_account_ids to use the
attribute rather than the helper function

fixes VICE-662
flag=none

/ ---- ---- \
| Test Plan |
\ ---- ---- /

- specs pass

Change-Id: If0397da0350ba74d336ee28de14abfddf69b35bb
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/243998
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>

app/models/conversation.rb

@@ -75,7 +75,7 @@ class Conversation < ActiveRecord::Base
         :workflow_state => 'read',
         :has_attachments => has_attachments?,
         :has_media_objects => has_media_objects?,
-        :root_account_ids => self.root_account_ids.join(',')
+        :root_account_ids => read_attribute(:root_account_ids)
     }.merge(options)
     ConversationParticipant.bulk_insert(user_ids.map{ |user_id|
       options.merge({:user_id => user_id})
@@ -654,7 +654,7 @@ class Conversation < ActiveRecord::Base
   def update_root_account_ids
     if root_account_ids_changed?
       # ids must be sorted for the scope to work
-      latest_ids = root_account_ids.sort.join(',')
+      latest_ids = read_attribute(:root_account_ids)
       %w[conversation_participants conversation_messages conversation_message_participants].each do |assoc|
         scope = self.send(assoc).where("#{assoc}.root_account_ids IS DISTINCT FROM ?", latest_ids).limit(1_000)
         until scope.update_all(root_account_ids: latest_ids) < 1_000; end

lib/conversation_helper.rb

@@ -21,6 +21,8 @@ module ConversationHelper
     write_attribute(:root_account_ids, conversation&.root_account_ids&.sort&.join(','))
   end
 
+  # Note: all ids are relative to the birth shard, if you are going to consume these you should do
+  # `Shard.birth.activate! do`
   def root_account_ids
     (read_attribute(:root_account_ids) || '').split(',').map(&:to_i).sort
   end