models.py

from django.db import models

# Create your models here.

class Category(models.Model):
    parent = models.ForeignKey('self', null = True, blank = True)
    slug = models.SlugField()
    level = models.SmallIntegerField(editable = False)
    order = models.SmallIntegerField(editable = False)

    def __unicode__(self):
        return u"%s" % '-'*self.level+' '+self.slug

    def save(self):
        if self.parent == None:
            self.level = 0
        else:
            self.level = self.parent.level + 1

        self.order =  0

        super(Category, self).save()

    class Meta:
        ordering = ('order', 'slug')


class Tag(models.Model):
    parent = models.ForeignKey(Category, null = True, blank = True)
    slug = models.SlugField()

    def __unicode__(self):
        return u"%s" % self.slug

class TestModel(models.Model):
    slug = models.SlugField()
    tags = models.ManyToManyField('Tag')