[CE] Support multiple assignees for merge requestsosw-multi-assignees-merge-requests

Backports https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10161
(code out of ee/ folder).

2 files changed, 6 insertions, 2 deletions

diff --git a/app/mailers/emails/merge_requests.rb b/app/mailers/emails/merge_requests.rb
index 9ba8f92fcbf..63148831a24 100644
--- a/app/mailers/emails/merge_requests.rb
+++ b/app/mailers/emails/merge_requests.rb
@@ -24,10 +24,12 @@ module Emails
     end
 
     # rubocop: disable CodeReuse/ActiveRecord
-    def reassigned_merge_request_email(recipient_id, merge_request_id, previous_assignee_id, updated_by_user_id, reason = nil)
+    def reassigned_merge_request_email(recipient_id, merge_request_id, previous_assignee_ids, updated_by_user_id, reason = nil)
       setup_merge_request_mail(merge_request_id, recipient_id)
 
-      @previous_assignee = User.find_by(id: previous_assignee_id) if previous_assignee_id
+      @previous_assignees = []
+      @previous_assignees = User.where(id: previous_assignee_ids) if previous_assignee_ids.any?
+
       mail_answer_thread(@merge_request, merge_request_thread_options(updated_by_user_id, recipient_id, reason))
     end
     # rubocop: enable CodeReuse/ActiveRecord
diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index efa1233b434..0b740809f30 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -4,6 +4,7 @@ class Notify < BaseMailer
   include ActionDispatch::Routing::PolymorphicRoutes
   include GitlabRoutingHelper
   include EmailsHelper
+  include IssuablesHelper
 
   include Emails::Issues
   include Emails::MergeRequests
@@ -24,6 +25,7 @@ class Notify < BaseMailer
   helper MembersHelper
   helper AvatarsHelper
   helper GitlabRoutingHelper
+  helper IssuablesHelper
 
   def test_email(recipient_email, subject, body)
     mail(to: recipient_email,