gistfile1.rb

class Order < ActiveRecord::Base
  has_one :payment
  def complete
    payment.complete
  end
end

class Payment < ActiveRecord::Base
  belongs_to :gateway_transaction
  def complete
    gateway_transaction.complete
  end
end

class GatewayTransaction < ActiveRecord::Base
  has_one :payment
  def complete
    errors.add(:base, 'Error!')
  end
end

order = Order.new(payment: Payment.new(source: GatewayTransaction.new))
order.complete
order.save #returns true