Make discussion mail References all notes in the discussion

When a note is part of a discussion, the email sent out will be
`In-Reply-To` the previous note in that discussion. It also
`References` all the previous notes in that discussion, and the
original issue.

Closes gitlab-org/gitlab-ce#36054.

2 files changed, 9 insertions, 8 deletions

diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index 250583d2d28..ec886e993c3 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -119,8 +119,8 @@ class Notify < BaseMailer
       headers['Reply-To'] = address
 
       fallback_reply_message_id = "<reply-#{reply_key}@#{Gitlab.config.gitlab.host}>".freeze
-      headers['References'] ||= ''
-      headers['References'] << ' ' << fallback_reply_message_id
+      headers['References'] ||= []
+      headers['References'] << fallback_reply_message_id
 
       @reply_by_email = true
     end
@@ -158,8 +158,8 @@ class Notify < BaseMailer
 
   def mail_answer_note_thread(model, note, headers = {})
     headers['Message-ID'] = message_id(note)
-    headers['In-Reply-To'] = message_id(note.replies_to)
-    headers['References'] = message_id(model)
+    headers['In-Reply-To'] = message_id(note.references.last)
+    headers['References'] = note.references.map { |ref| message_id(ref) }
 
     headers['X-GitLab-Discussion-ID'] = note.discussion.id if note.part_of_discussion?
 
diff --git a/app/models/note.rb b/app/models/note.rb
index ea31bc45c61..184fbd5f5ae 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -360,13 +360,14 @@ class Note < ActiveRecord::Base
     end
   end
 
-  def replies_to
+  def references
+    refs = [noteable]
+
     if part_of_discussion?
-      previous_note = discussion.notes.take_while { |n| n.id < id }.last
-      return previous_note if previous_note
+      refs += discussion.notes.take_while { |n| n.id < id }
     end
 
-    noteable
+    refs
   end
 
   def expire_etag_cache