title: Create a field whose value is a calculation of other fields' values
body: <pre><code>class PO(models.Model)
      qty = models.IntegerField(null=True)
      cost = models.IntegerField(null=True)
      total = qty * cost
  </code></pre>
  
  <p>How will I solve <code>total = qty * cost</code> above. I know it will cause an error, but have no idea of how to deal with this.</p>