django/tests/modeltests/m2o_recursive/models.py

"""
11. 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', blank=True, null=True, related_name='child_set')

    def __unicode__(self):
        return self.name

__test__ = {'API_TESTS':"""
# Create a few Category objects.
>>> r = Category(id=None, name='Root category', parent=None)
>>> r.save()
>>> c = Category(id=None, name='Child category', parent=r)
>>> c.save()

>>> r.child_set.all()
[<Category: Child category>]
>>> r.child_set.get(name__startswith='Child')
<Category: Child category>
>>> print r.parent
None

>>> c.child_set.all()
[]
>>> c.parent
<Category: Root category>
"""}