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

Model source code

from django.core import meta

class Category(meta.Model):
    name = meta.CharField(maxlength=20)
    parent = meta.ForeignKey('self', null=True, related_name='child')
    class META:
        module_name = 'categories'

    def __repr__(self):
        return self.name

API reference

Category objects have the following methods:

• add_child()
• delete()
• get_child()
• get_child_count()
• get_child_list()
• get_parent()
• save()

Sample API usage

This sample code assumes the above model has been saved in a file examplemodel.py.

>>> from django.models.examplemodel import categories

# Create a few Category objects.
>>> r = categories.Category(id=None, name='Root category', parent=None)
>>> r.save()
>>> c = categories.Category(id=None, name='Child category', parent=r)
>>> c.save()

>>> r.get_child_list()
[Child category]
>>> r.get_child(name__startswith='Child')
Child category
>>> r.get_parent()
Traceback (most recent call last):
    ...
CategoryDoesNotExist

>>> c.get_child_list()
[]
>>> c.get_parent()
Root category