title: Django save() always create a new object
body: <p>I have this model in my app which is meant to auto-generate it's primary Key based on a method added in the save().<br>
  However, for each object, I will be expected to make updates of certain fields. Right now, anytime I make an update on the admin side (testing use cases) it instead creates a new record of the PK instead of updating the existing one. Any thoughts on how to remedy this? </p>
  
  <pre><code>class DeploymentTask(models.Model):
      deployment_id = models.CharField(
          'Deployment Task ID', primary_key=True, max_length=25, editable=False)
      title = models.CharField(max_length=100)
      current_status = FSMField('Current Status',
                                default=STATES[0], choices=STATES)
      site_id = models.ForeignKey(
          Site, related_name='+', on_delete=models.CASCADE)
      created_at = models.DateTimeField(auto_now_add=True)
      updated_at = models.DateTimeField(auto_now=True)
  
      refuel_record = models.ManyToManyField(RefuelRecord)
  
      def __str__(self):
          """String for representing the Model object."""
          return self.deployment_id
  
      class Meta:
          db_table = 'rm_deployment_task'
          verbose_name_plural = 'Deployment Tasks'
  
      def get_absolute_url(self):
          return reverse('deployment_id-view', args=[str(self.deployment_id)])
  
      def save(self):
          today = datetime.datetime.now()
          ticket_count = DeploymentTask.objects.filter(
              created_at__year=today.year, created_at__month=today.month).count() + 1
          new_task_id = 'DPT-' + str(str(datetime.date.today().year)) + str(
              datetime.date.today().month).zfill(2) + str(
              datetime.date.today().day).zfill(2) + '-' + str(ticket_count).zfill(6)
          self.deployment_id = new_task_id
          super(DeploymentTask, self).save()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/QMbpv.png" rel="nofollow noreferrer">enter image description here</a></p>