Touch `notifications_updated_at` after marking entries as read

app/controllers/notifications_controller.rb

@@ -48,10 +48,13 @@ class NotificationsController < ApplicationController
                 .count(:target_type)
   end
 
+  # rubocop:disable Rails/SkipsModelValidations
   def mark_notifications_as_read
     # using .dup to not modify @notifications -- useful in tests
-    @notifications&.dup&.update_all(new: false) # rubocop:disable Rails/SkipsModelValidations
+    @notifications&.dup&.update_all(new: false)
+    current_user.touch(:notifications_updated_at)
   end
+  # rubocop:enable Rails/SkipsModelValidations
 
   def cursored_notifications_for(type:, last_id:, size: nil)
     cursor_params = { last_id: last_id, size: size }.compact

spec/controllers/notifications_controller_spec.rb

@@ -3,9 +3,12 @@
 require "rails_helper"
 
 describe NotificationsController do
+  include ActiveSupport::Testing::TimeHelpers
+
   describe "#index" do
     subject { get :index, params: { type: :new } }
 
+    let(:original_notifications_updated_at) { 1.day.ago }
     let(:user) { FactoryBot.create(:user) }
 
     before do
@@ -38,6 +41,13 @@ describe NotificationsController do
       it "marks notifications as read" do
         expect { subject }.to change { Notification.for(user).where(new: true).count }.from(2).to(0)
       end
+
+      it "updates the the timestamp used for caching" do
+        user.update(notifications_updated_at: original_notifications_updated_at)
+        travel 1.second do
+          expect { subject }.to change { user.reload.notifications_updated_at }.from(original_notifications_updated_at).to(Time.now.utc)
+        end
+      end
     end
   end