Django using ForeignKey for tree relationship, but avoiding self referencing

Question

I have a model on Django that forms a tree like structure:

class Person(models.Model):
    name = models.CharField(max_length = 255)

    parent = models.ForeignKey(
        'self', 
        on_delete=models.DO_NOTHING, 
        blank = True, 
        null = True, 
        limit_choices_to=????,
)

def __str__(self):
        return self.name

So far everything works good, admin generates a form and I am able to add new persons to the database. My problem is on the parent relationship: how can I avoid a self reference? I am trying to use the limit_choices_to, but so far I could not figure out how to use it to solve this.

Thank you