So the final call is:

  @order.withdraw(amount)

In this case it’s now a bit unclear *what* I’m withdrawing.

So Customer becomes:

  class Customer
    has_many :orders
    has_one :credit_card
    has_one :bank_account
    def withdraw(amount)
      if pay_by_card?
        credit_card.withdraw(amount)
      elsif pay_by_account?
        bank_account.withdraw(amount)
      end
    end
  end

Or if there is only credit cards supported the chain can just be:

  class Customer
    has_many :orders
    has_one :credit_card
    delegate :withdraw, :to => :credit_card
  end

Where does the pay_by_card come from?