rails-docstore/app/models/document.rb

class Document < ApplicationRecord
  belongs_to :user
  belongs_to :category
  has_and_belongs_to_many :tags

  has_attached_file :doc,
  {
    adapter_options: { hash_digest: Digest::SHA256 },
    url: "/system/:class/:attachment/:id_partition/:style/:hash.:extension",
    hash_secret: Rails.application.secrets.secret_key_base
  }
  validates_attachment_content_type :doc, content_type: 'application/pdf'
end