transacton.rb

class Transaction
  attr_accessor :amount, :parent

  def initialize(amount, parent)
    self.amount = amount
    self.parent = parent
  end
  
  def each
    # while there are parent transactions, keep yielding them
    current_transaction = self
    yield current_transaction
    while current_transaction.parent
      current_transaction = current_transaction.parent
      yield current_transaction
    end
  end
  include Enumerable
end

t1 = Transaction.new(100, nil)
t2 = Transaction.new(250, t1)
t3 = Transaction.new(300, t2)

current = t3
current.inject(0) {|total, tr| total + tr.amount}