Django documentation

11. Relating an object to itself, many-to-one

This example is for Django's SVN release, which can be significantly different from previous releases. Get old examples here: 0.96, 0.95.

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.

Model source code

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

Sample API usage

This sample code assumes the above model has been saved in a file mysite/models.py. 

>>> from mysite.models import Category

# 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>

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