class Order < ActiveRecord::Base
  ...
  def validate_credit_card_number
    unless self.payment_type.validate_card_number(
      :credit_card_number,
      :credit_expiration_month,
      :credit_expiration_year)
    errors.add('credit_card_number', 'niepoprawny numer karty')
  end
  def validate
    validate_credit_card_number
  end
end

