title: Detect if a model has changed before calling save in Django
body: <p>I have a database model that is being updated based on changes in remote data (via an HTML scraper).</p>
  
  <p>I want to maintain a field called <code>changed</code> - a timestamp denoting when the last time that model's values changed from what they were previously (note that this is different from <code>auto_now</code> as these fields are updated every time a model's <code>save</code> method is called).</p>
  
  <p>Here is my question:</p>
  
  <p>In a model's <code>save</code> method, is there a straightforward way to detect if a model instance's current values are different from the values in the database? Or, are there any alternative methods to easily maintain a <code>changed</code> timestamp?</p>