Enterprise Rails

Factory Classes

Let’s return to the Payment class. It’s a very sad class, completely empty and with next to no purpose; however, we can give it a purpose by turning it into a factory class, utilizing another pattern in the famous Gang of Four Design Patterns (Addison-Wesley) by Erich Gamma et al. and making our MTI solution even more powerful and DRY.

A factory class is a class that has a constructor that returns instances of the correct subclass based on the inputs. Currently, if we have web forms with a radio selector for the payment type (Paypal, credit, or promotional), we’d have to write a case statement in every place we’re processing the input in order to create an instance of the correct type. Same goes for the credit card type (Visa, American Express, or MasterCard).

Instead, we can localize this logic in the Payment class itself. Any code that needs a new payment object can pass in the appropriate information to the factory method and out will come an object of the correct type.

In the following code, we define a constructor, new_payment, for the Payment class:

class Payment < ActiveRecord::Base
  def self.new_payment(payment_type, credit_card_type)
    case type
    when 'paypal'
      PaypalPayment.new
    when 'promotional'
      PromotionalPayment.new
    when 'credit_card'
      CreditCardPayment.new_payment credit_card_type
    end
  end
end

Notice that if the payment is a credit card payment, we defer to a constructor within the CreditCardPayment class. That constructor might look like this:

class CreditCardPayment ...