title: Model field based on other fields?
body: <p>Can I have a model field be based on other fields? For example:</p>
  
  <pre><code>class Foo(models.Model):
      x = models.PositiveSmallIntegerField()
      y = models.PositiveSmallIntegerField()
      z = models.PositiveSmallIntegerField()
  
      score = models.PositiveSmallIntegerField(default=x+y+z)
  </code></pre>