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

app/services/notification_service.rb

@@ -376,10 +376,10 @@ class NotificationService
   end
 
   def reassign_resource_email(target, project, current_user, method)
-    previous_assignee_id = previous_record(target, "assignee_id")
+    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, action: :reassign, extra_recipients: [previous_assignee])
+    recipients = build_recipients(target, project, current_user, action: :reassign, previous_assignee: previous_assignee)
 
     recipients.each do |recipient|
       mailer.send(
@@ -400,11 +400,9 @@ class NotificationService
     end
   end
 
-  def build_recipients(target, project, current_user, action: nil, extra_recipients: nil)
+  def build_recipients(target, project, current_user, action: nil, previous_assignee: 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)
 
@@ -412,6 +410,7 @@ class NotificationService
     # new assignee to the list of recipients after we rejected users with
     # the "on mention" notification level
     if action == :reassign
+      recipients << previous_assignee if previous_assignee
       recipients << target.assignee
     end
 

spec/services/notification_service_spec.rb

@@ -227,6 +227,46 @@ describe NotificationService, services: true do
     end
 
     describe :reassigned_issue do
+      it 'emails new assignee' do
+        notification.reassigned_issue(issue, @u_disabled)
+
+        should_email(issue.assignee)
+        should_email(@u_watcher)
+        should_email(@u_participant_mentioned)
+        should_email(@subscriber)
+        should_not_email(@unsubscriber)
+        should_not_email(@u_participating)
+        should_not_email(@u_disabled)
+      end
+
+      it 'emails previous assignee even if he has the "on mention" notif level' do
+        issue.update_attribute(:assignee, @u_mentioned)
+        issue.update_attributes(assignee: @u_watcher)
+        notification.reassigned_issue(issue, @u_disabled)
+
+        should_email(@u_mentioned)
+        should_email(@u_watcher)
+        should_email(@u_participant_mentioned)
+        should_email(@subscriber)
+        should_not_email(@unsubscriber)
+        should_not_email(@u_participating)
+        should_not_email(@u_disabled)
+      end
+
+      it 'emails new assignee even if he has the "on mention" notif level' do
+        issue.update_attributes(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)
+        should_email(@subscriber)
+        should_not_email(@unsubscriber)
+        should_not_email(@u_participating)
+        should_not_email(@u_disabled)
+      end
+
       it 'emails new assignee' do
         issue.update_attribute(:assignee, @u_mentioned)
         notification.reassigned_issue(issue, @u_disabled)