title: Calling a model function in django-admin
body: <p>is there any method I can call a model function in admin.py for eg suppose I have a model say</p>
  
  <pre><code>class A(models.Model):
      first = models.IntegerField()
      second = models.IntegerField()
      total = models.IntegerField()
  
      def Total_amount(self):
          self.total+=first
          self.total+=second
  </code></pre>
  
  <p>Now I want that whenever I do something from admin side into first and second it automatically must reflect into total's value.How can I do that I mean I can do something in ModelAdmin class but its not working from my side.</p>