title: New to python django, how to auto populate a field?
body: <p>I have two fields in my model.py one has multi choice drop down and one that is empty. What I would like to have is that if the user select "Gas" from the menu for type, I would like the amount field to get auto populated with distance * 2
  Can I do that?</p>
  
  <pre><code>CHOICE = (
      ('Meal', 'Meal'),
      ('Gas', 'Gas'),
      )
  type = models.CharField(max_length=10, choices=CHOICE)
  distance = models.CharField(max_length=100)
  amount = models.CharField(max_length=100)
  </code></pre>
  
  <p>Thanks in advance.</p>