#3

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