gift_certificate.rb

  def self.use_certificate(code, cart)
    return false if cart.nil?
    return cart.set_gift(0, nil) if code.blank?
    return false if cart.gift_code == code

    certificate = self.certificate(code).first
    unless certificate.blank? || certificate.state == 'in_cart'
      unless certificate.state == 'empty'
        total = cart.total + cart.gift_discount
        if certificate.amount < total
          cart.set_gift(certificate.amount, certificate.gift_code)
        else
          cart.set_gift(total, certificate.gift_code)
        end
      else
        cart.set_gift(0, nil)
      end
    else
      cart.set_gift(0, nil)
      return false
    end
  end