title: django - update date automatically after a value change
body: <p>In the following django model:</p>
  
  <pre><code>class MyModel(models.Model):
      published = models.BooleanField(default=False)
      pub_date = models.DateTimeField('date published')
  </code></pre>
  
  <p>I want the pub_date field to be automatically updated with the current date, every time the published field changes to True.</p>
  
  <p>What is the smart way?</p>
  
  <p>Thanks.</p>