arrays.rb

class TransactionHistory
  def transactions
    adapters = [ChicagoTransactionAdapter, DallasTransactionAdapter]
    transactions = []
    adapters.each do |adapter|
      transactions += adapter.fetch_transactions
    end
  end
end

class ChicagoTransactionAdapter; end
class DallasTransactionAdapter; end

open_closed.rb

class TransactionHistory
  def transactions
    transactions = []
    TransactionAdapter.adapters.each do |adapter|
      transactions += adapter.fetch_transactions
    end
  end
end

class ChicagoTransactionAdapter
  include TransactionAdapter
end

class DallasTransactionAdapter
  include TransactionAdapter
end

module TransactionAdapter
  @@adapters = []
  
  def included(other)
    @@adapters << other
  end
  
  def self.adapters
    @@adapters
  end
end