Touch merge request updated_at when assignees changed

app/models/merge_request_assignee.rb

 # frozen_string_literal: true
 
 class MergeRequestAssignee < ApplicationRecord
-  belongs_to :merge_request
+  belongs_to :merge_request, touch: true
   belongs_to :assignee, class_name: "User", foreign_key: :user_id, inverse_of: :merge_request_assignees
 
   validates :assignee, uniqueness: { scope: :merge_request_id }

changelogs/unreleased/pherlihy-master-patch-95213.yml

+---
+title: Change merge request updated_at when assignees are changed
+merge_request: 41030
+author: Patrick Herlihy
+type: fixed

spec/features/merge_request/user_assigns_themselves_spec.rb

@@ -21,6 +21,10 @@ RSpec.describe 'Merge request > User assigns themselves' do
       expect(page).to have_content '2 issues have been assigned to you'
     end
 
+    it 'updates updated_by', :js do
+      expect { click_button 'assign yourself' }.to change { merge_request.reload.updated_at }
+    end
+
     it 'returns user to the merge request', :js do
       click_link 'Assign yourself to these issues'