Ensure inbox caching timestamp gets updated when answering questions and returning to inbox

app/models/answer.rb

@@ -20,6 +20,7 @@ class Answer < ApplicationRecord
 
   after_create do
     Inbox.where(user: self.user, question: self.question).destroy_all
+    user.touch :inbox_updated_at # rubocop:disable Rails/SkipsModelValidations
 
     Notification.notify self.question.user, self unless self.question.user == self.user or self.question.user.nil?
     Subscription.subscribe self.user, self

spec/controllers/ajax/answer_controller_spec.rb

@@ -4,6 +4,8 @@
 require "rails_helper"
 
 describe Ajax::AnswerController, :ajax_controller, type: :controller do
+  include ActiveSupport::Testing::TimeHelpers
+
   let(:question) { FactoryBot.create(:question, user: FactoryBot.build(:user, privacy_allow_stranger_answers: asker_allows_strangers)) }
   let(:asker_allows_strangers) { true }
 
@@ -327,6 +329,14 @@ describe Ajax::AnswerController, :ajax_controller, type: :controller do
           user.save
           expect { subject }.to(change { Inbox.where(question_id: answer.question.id, user_id: user.id).count }.by(1))
         end
+
+        it "updates the inbox caching timestamp for the user who answered" do
+          initial_timestamp = 1.day.ago
+          answer.user.update(inbox_updated_at: initial_timestamp)
+          freeze_time do
+            expect { subject }.to(change { answer.user.reload.inbox_updated_at }.from(initial_timestamp).to(DateTime.now))
+          end
+        end
       end
 
       context "when the answer exists and was not made by the current user" do