Create todos only for newly mentioned users on epics

ee/app/services/epics/update_service.rb

@@ -3,11 +3,14 @@ module Epics
     def execute(epic)
       update(epic)
 
-      # TODO: old_mentioned_users
-      # TODO: move to handle_changes method
-      todo_service.update_epic(epic, current_user, [])
-
       epic
     end
+
+    def handle_changes(epic, options)
+      old_associations = options.fetch(:old_associations, {})
+      old_mentioned_users = old_associations.fetch(:mentioned_users, [])
+
+      todo_service.update_epic(epic, current_user, old_mentioned_users)
+    end
   end
 end

ee/spec/services/epics/update_service_spec.rb

@@ -6,6 +6,9 @@ describe Epics::UpdateService do
   let(:epic) { create(:epic, group: group) }
 
   describe '#execute' do
+    before do
+      stub_licensed_features(epics: true)
+    end
     def update_epic(opts)
       described_class.new(group, user, opts).execute(epic)
     end
@@ -56,5 +59,21 @@ describe Epics::UpdateService do
         expect(note.noteable).to eq(epic)
       end
     end
+
+    context 'todos' do
+      let(:mentioned1) { create(:user) }
+      let(:mentioned2) { create(:user) }
+
+      before do
+        group.update(visibility: Gitlab::VisibilityLevel::PUBLIC)
+        epic.update(description: "FYI: #{mentioned1.to_reference}")
+      end
+
+      it 'creates todos for only newly mentioned users' do
+        expect {
+          update_epic(description: "FYI: #{mentioned1.to_reference} #{mentioned2.to_reference}")
+        }.to change { Todo.count }.by(1)
+      end
+    end
   end
 end