t.rb

Transaction = Struct.new(:amount, :parent)

t1 = Transaction.new 1, nil
t2 = Transaction.new 2, t1
t3 = Transaction.new 4, t2
t4 = Transaction.new 8, nil

TransactionEnumerator = Struct.new(:ary) do
  include Enumerable

  def each
    ary.each do |t|
      yield t
      yield t while t = t.parent
    end
  end
end

ts = TransactionEnumerator.new [t3, t4]
puts ts.inject(0) { |total, transaction| total + transaction.amount }