Class-Oriented example

models.rb

class Department < ActiveRecord::Base
  has_one :stock
end

class Stock < ActiveRecord::Base
  belongs_to :department

  def update
    update_attribute :value, Legacy::Department.find(department.id).stock_value
  end
end

class Legacy::Department < ActiveResource::Base
  # stuff here
end

stock_controller.rb

class StocksController
  def update
    Stock.where(:department_id => params[:department_id]).first.udpate
  end
end