title: Return function as a field on django model
body: <p>I have a model which have a function to calculate the difference between two fields
  Example:</p>
  
  <pre><code>Class MyModel(models.Model):
      fieldA = models.FloatField()
      fieldB = models.FloatField()
  
      def delta(self):
          return self.fieldA - self.fieldB
  </code></pre>
  
  <p>Id like to use this model in a GenericView. I can use the function delta as an extraContext but also id like to include the sum of all Delta results in the template, in this case I have to make an aggregate but again since delta is not a DB Field nor a Model Field I cant use it in an aggregate function.</p>
  
  <p>How can accomplish this?</p>