#3

backend/app/controllers/thread_posts_controller.rb

@@ -24,7 +24,12 @@ class ThreadPostsController < ApplicationController
   def create
     post = @thread.posts.new(post_params)
     if post.save
-      render json: post, status: :created
+      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)
+          else
+            nil
+          end)), status: :created
     else
       render json: { errors: post.errors.full_messages }, status: :unprocessable_entity
     end
@@ -37,6 +42,8 @@ class ThreadPostsController < ApplicationController
   end
 
   def post_params
-    params.require(:post).permit(:name, :body, :password)
+    params.permit(:name, :message, :password, :image).transform_values { |v|
+      v.presence
+    }
   end
 end

backend/app/models/post.rb

@@ -2,7 +2,16 @@ class Post < ApplicationRecord
   belongs_to :thread, class_name: 'Topic', foreign_key: :thread_id
   has_one_attached :image
 
+  before_create :set_post_no
+
   has_secure_password validations: false
 
   scope :active, -> { where deleted_at: nil }
+
+  private
+
+  def set_post_no
+    max_no = Post.where(thread_id:).maximum(:post_no) || 0
+    self.post_no = max_no + 1
+  end
 end