"""

Relating an object to itself, many-to-one

To define a many-to-one relationship between a model and itself, use

``ForeignKey('self', ...)``.

In this example, a ``Category`` is related to itself. That is, each

``Category`` has a parent ``Category``.

Set ``related_name`` to designate what the reverse relationship is called.

"""

from django.db import models

class Category(models.Model):

    name = models.CharField(max_length=20)

    parent = models.ForeignKey(

        "self", models.SET_NULL, blank=True, null=True, related_name="child_set"

    )

    def __str__(self):

        return self.name

class Person(models.Model):

    full_name = models.CharField(max_length=20)

    mother = models.ForeignKey(

        "self", models.SET_NULL, null=True, related_name="mothers_child_set"

    )

    father = models.ForeignKey(

        "self", models.SET_NULL, null=True, related_name="fathers_child_set"

    )

    def __str__(self):

        return self.full_name