title: how to override method update in Django form?
body: <p>i have model like that :</p>
  <pre><code>class BrokerCredential(models.Model):
      api_key = models.TextField(blank=True, null=True)
      age = models.IntegerField(blank=True, null=True)
  
      def save(self, *args, **kwargs):
          self.api_key = self.api_key * 3
      super(BrokerCredential, self).save(*args, **kwargs)
  </code></pre>
  <p>I override method 'save()' because I wanted every time and everywhere when I need to save record, this method manipulates my input.</p>
  <p>But there is a problem, whenever I update my model ( consider changing the 'age' field), it multiples 'api_key' again.</p>
  <p>I need a way that saves method works only when I add a new record, not every time I update my model instance.</p>