Increment id within a parent entity in Django model

I have this problem where I have a parent model User that is one-to-many linked to another model say Device.

What I want to do is within each user, I want to create and increment an id for each device he own.

Something like user A have two devices so his user_device_id will be 1 and 2. user B have one device so his user_device_id will be 1. vice versa.

I have a current working solution but I am looking for an alternative solution.

The Device model currently implemented which overrides save method:

class Device(models.Model):

    user = models.ForeignKey(User, on_delete=models.CASCADE)
    user_device_id = models.PositiveIntegerField(default=0, null=false)

    def save(self, *args, **kwargs):
        try:
            within_user_device_id = Device.objects.filter(user=self.user).latest('user_device_id').user_device_id
            self.user_device_id = within_user_device_id + 1
        except Device.DoesNotExist:
            self.user_device_id = 1

For the neater method, in my mind, to create a function that gets the current instance of user and use the default in models.PositiveIntegerField to increment this value.

The problem is I cannot get refer the current instance and call the function in default.

class Device(models.Model):

    def increment_within_user_device_id(self):
        within_user_device_id = Device.objects.filter(user=self.user).latest('user_device_id').user_device_id
        if not within_user_device_id:
            return 1
        else:
            within_user_device_id += 1
            return within_user_device_id

    user = models.ForeignKey(User, on_delete=models.CASCADE)
    # I cannot pass the function in default
    user_device_id = models.PositiveIntegerField(default=increment_with_user_device_id(self), null=false)

Any ideas?