code: def save(self, *args, **kwargs):
          
          try:
              prev_instance = self.__class__.objects.get(pk=self.pk)
          except self.DoesNotExist:
              prev_instance = None
  
          if prev_instance is None or (not prev_instance.is_active and self.is_active or
                                       self.schedule != prev_instance.schedule):
              self.update_next_trigger_at()
  
          super(ScheduleMixin, self).save(*args, **kwargs)
docs: Updates next_trigger_at field if:
           - instance become active
           - instance.schedule changed
           - instance is new