rail 3+ activemerchant: Get data from association class

Hi

I'm using ActiveMerchant with Authorize.net and already setup 2 models
Order and Transaction. (http://railscasts.com/episodes/145-integrating-
active-merchant)

I would like to get error message from the transaction if error
occurred then output in flash message. Here is my code:

#models/order_transaction.rb

class OrderTransaction < ActiveRecord::Base
  belongs_to :order
  serialize :params

  def response=(response)
    self.success = response.success?
    self.authorization = response.authorization
    self.message = response.message
    self.params = response.params
  rescue ActiveMerchant::ActiveMerchantError => e
    self.success = false
    self.authorization = nil
    self.message = e.message
    self.params = {}
  end
end

How do I get those data from OrderTransaction class such as message,
success... in order_controller?

Thank you!