btrc-hub/backend/app/models/post.rb

class Post < ApplicationRecord
  belongs_to :parent, class_name: 'Post', optional: true, foreign_key: 'parent_id'
  belongs_to :uploaded_user, class_name: 'User', optional: true
  has_many :post_tags, dependent: :destroy
  has_many :tags, through: :post_tags
  has_many :user_post_views, dependent: :destroy
  has_one_attached :thumbnail

  def as_json options = { }
    super(options).merge({ thumbnail: thumbnail.attached? ?
                                      Rails.application.routes.url_helpers.rails_blob_url(
                                          thumbnail, only_path: false) :
                                      nil })
  end
end