title: Django set field values dynamically based on other fields
body: <p>I'm trying to set a field default value based on other field selection that is foreign.
  Basically, these are the classes:</p>
  
  <pre><code>class Product(models.Model):
          description = models.CharField('Description', max_length=200)
          price = models.FloatField('Price')
  
  class Sell(models.Model):
          product = models.ForeignKey(Product)
          price = models.FloatField('Price')
  </code></pre>
  
  <p>Each "Product" has a default price (or suggested price), so when the user, in Admin pages, want to add a new Sell and he/she choose a product, I need to copy dynamically from Product.price to Sell.price the suggested price.
  I cannot use "save" method because the user can change at that moment.</p>
  
  <p>Is it necessary to use JavaScript explicitly? or Is there an elegant way in Django to do that?</p>