#5

backend/app/controllers/thread_posts_controller.rb

@@ -24,6 +24,9 @@ class ThreadPostsController < ApplicationController
   def create
     post = @thread.posts.new(post_params)
     if post.save
+      post.thread.tap { |thread|
+        thread.updated_at = post.created_at
+      }.save!
       render json: post.as_json.merge(image_url: (
           if post.image.attached?
             Rails.application.routes.url_helpers.rails_blob_url(post.image, only_path: true)