"""

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(maxlength=20)

    parent = models.ForeignKey('self', null=True, related_name='child_set')

    def __str__(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>

"""}