title: Rails 2. Calling virtual attribute's setter by update_attributes
body: <p>I have one extra attribute in my form (:pagesize) which is not in database. 
  There is also a setter for it:</p>
  
  <pre><code>def pagesize= pagesize
    self.preferences["pagesize"] = pagesize
  end
  </code></pre>
  
  <p>I want 'update_attributes' method to call that setter.
  What is the best type for :pagesize attribute? attr_accessor, attr_writer...</p>