title: Isolate the initial quantity of articles on create and update
body: <p>When I create an article I set a quantity.</p>
  
  <p>Each time an article is sold, its quantity is decreased by a method.
  I need to keep my initial stock, the number of sales and the remaining stock...</p>
  
  <p>So my question is:</p>
  
  <p>How can I keep the initial quantity, and the quantity I could add on update?</p>
  
  <pre><code># article.rb
  before_create :quantity_on_create
  before_update :quantity_on_update
  
  def quantity_on_create
    self.quantity
  end
  
  def quantity_on_update
      quantity_on_create += self.quantity
  end
  </code></pre>
  
  <p><code>quantity_on_create</code> remains nil ?</p>