Make a system comment when issue milestone changed

app/models/note.rb

@@ -90,6 +90,22 @@ class Note < ActiveRecord::Base
       create(note_options, without_protection: true)
     end
 
+    def create_milestone_change_note(noteable, project, author, milestone)
+      body = if milestone.nil?
+               '_Milestone removed_'
+             else
+               "_Milestone changed to #{milestone.title}_"
+             end
+
+      create({
+        noteable: noteable,
+        project: project,
+        author: author,
+        note: body,
+        system: true
+      }, without_protection: true)
+    end
+
     def create_assignee_change_note(noteable, project, author, assignee)
       body = assignee.nil? ? '_Assignee removed_' : "_Reassigned to @#{assignee.username}_"
 

app/services/issues/base_service.rb

@@ -10,5 +10,9 @@ module Issues
     def execute_hooks(issue)
       issue.project.execute_hooks(issue.to_hook_data, :issue_hooks)
     end
+
+    def create_milestone_note(issue)
+      Note.create_milestone_change_note(issue, issue.project, current_user, issue.milestone)
+    end
   end
 end

app/services/issues/update_service.rb

@@ -13,6 +13,10 @@ module Issues
       if params.present? && issue.update_attributes(params)
         issue.reset_events_cache
 
+        if issue.previous_changes.include?('milestone_id')
+          create_milestone_note(issue)
+        end
+
         if issue.previous_changes.include?('assignee_id')
           notification_service.reassigned_issue(issue, current_user)
           create_assignee_note(issue)