How to exclude existing rows for CharField(choices)?

Question

Django

There is a field, let's say:

CUSTOM_CHOICES = [
    ('0', '0'),
    ('1', '1'),
    ('2', '2'),
    ('3', '3'),
]
choice = models.CharField(max_length=2, choices=CUSTOM_CHOICES, default=0, unique=True, )

Because Since it is unique, it makes no sense to display values ​​that are in existing rows. I would like to know what solutions can be here?