title: How to check modified value in a model's before_save callback?
body: <p><strong>How can I read the value that a controller has set in a before_save callback?</strong></p>
  
  <p>Example:</p>
  
  <p>I have a model with a <code>url</code> field. Before saving, I want to check if the url was changed. If so, do some stuff with both the new and old url.</p>
  
  <p><strong>Is that possible?</strong></p>