contract_file.rb

class ContractFile < ActiveRecord::Base

  belongs_to :contract

  has_attached_file :data,
  :url => "/assets/paperclip/:parent_id/:attachment/:id/:style/:filename"
   #:url => '/public/system/:attachment/foto_:basename.:extension',
    #:url => '/assets/:id/foto_:basename.:extension'

  validates_attachment_size :data, :less_than => 5.megabytes
  
  Paperclip.interpolates :parent_id do |a, s|
    a.instance.contract.id.to_s
  end
end