title: rails before_save callback for new record only?
body: <p>Hi I would like to use my <code>before_save</code> call back only the first time. (When my activerecord is being saved for the first time. How do I do that?</p>
  
  <p>Currently in my callback I'm checking for attributes for .nil? before writing them. </p>
  
  <p>The purpose of this callback is to initialize an attribute from a value retrieved from an associated model. </p>
  
  <p>Any better way of doing this?</p>