gistfile1.txt

class Product < ActiveRecord::Base
  attr_accessible :name, :qty
  has_many :product_sales
end

class Sale < ActiveRecord::Base
  belongs_to :salesman
  has_many :product_sales
end

class SaleObserver < ActiveRecord::Observer
  def after_save(sale)
    sale.product.update_attribute(:qty,(sale.product.qty - sale.qty))
  end
end