Consider re-assign as a mention from a notification point of view

This will ensure new assignee gets an email even if his notif level is
"on mention".

app/services/notification_service.rb

@@ -379,7 +379,7 @@ class NotificationService
     previous_assignee_id = previous_record(target, "assignee_id")
     previous_assignee = User.find_by(id: previous_assignee_id) if previous_assignee_id
 
-    recipients = build_recipients(target, project, current_user, [previous_assignee])
+    recipients = build_recipients(target, project, current_user, action: :reassign, extra_recipients: [previous_assignee])
 
     recipients.each do |recipient|
       mailer.send(
@@ -400,22 +400,28 @@ class NotificationService
     end
   end
 
-  def build_recipients(target, project, current_user, extra_recipients = nil)
+  def build_recipients(target, project, current_user, action: nil, extra_recipients: nil)
     recipients = target.participants(current_user)
 
     recipients = recipients.concat(extra_recipients).compact.uniq if extra_recipients
 
     recipients = add_project_watchers(recipients, project)
     recipients = reject_mention_users(recipients, project)
-    recipients = reject_muted_users(recipients, project)
 
+    # Re-assign is considered as a mention of the new assignee so we add the
+    # new assignee to the list of recipients after we rejected users with
+    # the "on mention" notification level
+    if action == :reassign
+      recipients << target.assignee
+    end
+
+    recipients = reject_muted_users(recipients, project)
     recipients = add_subscribed_users(recipients, target)
     recipients = reject_unsubscribed_users(recipients, target)
 
     recipients.delete(current_user)
-    recipients = recipients.uniq
 
-    recipients
+    recipients.uniq
   end
 
   def mailer

spec/services/notification_service_spec.rb

@@ -227,9 +227,11 @@ describe NotificationService, services: true do
     end
 
     describe :reassigned_issue do
-      it 'should email new assignee' do
+      it 'emails new assignee' do
+        issue.update_attribute(:assignee, @u_mentioned)
         notification.reassigned_issue(issue, @u_disabled)
 
+        expect(issue.assignee).to be @u_mentioned
         should_email(issue.assignee)
         should_email(@u_watcher)
         should_email(@u_participant_mentioned)
@@ -238,6 +240,20 @@ describe NotificationService, services: true do
         should_not_email(@u_participating)
         should_not_email(@u_disabled)
       end
+
+      it 'does not email new assignee if they are the current user' do
+        issue.update_attribute(:assignee, @u_mentioned)
+        notification.reassigned_issue(issue, @u_mentioned)
+
+        expect(issue.assignee).to be @u_mentioned
+        should_email(@u_watcher)
+        should_email(@u_participant_mentioned)
+        should_email(@subscriber)
+        should_not_email(issue.assignee)
+        should_not_email(@unsubscriber)
+        should_not_email(@u_participating)
+        should_not_email(@u_disabled)
+      end
     end
 
     describe :close_issue do