our mptt category model that stores materialized URL for lookups

models.py

from fieldsignals import post_save_changed
from mptt.models import MPTTModel


class Category(MPTTModel):
    parent = models.ForeignKey('self', null=True, blank=True, db_index=True)
    name = models.CharField(max_length=50)
    slug = models.SlugField(max_length=50, db_index=True)
    url = models.CharField(max_length=256, unique=True, db_index=True)

    class MPTTMeta:
        order_insertion_by = ('slug',)

    class Meta:
        verbose_name_plural = _('categories')
        unique_together = (('parent', 'name'), ('parent', 'slug'))

    def save(self, *args, **kwargs):
        if not self.slug:
            self.slug = slugify(self.name)
        return super(Category, self).save(*args, **kwargs)

    def update_url(self):
        if self.parent:
            url = '%s/%s' % (self.parent.url, self.slug)
        else:
            url = self.slug

        if url == self.url:
            return

        # update url without trigger save() (avoid infinite recursion in post-save handler)
        self.url = url
        Category.objects.filter(pk=self.pk).update(url=url)

        # cascade to descendants
        cursor = connection.cursor()
        count = 1
        level = self.level
        while count:
            level += 1
            sql = """
            UPDATE "tags_category" SET url = (
                SELECT parent.url || '/' || tags_category.slug
                FROM "tags_category" parent
                WHERE parent.id = tags_category.parent_id
            )
            WHERE lft > %s AND rght < %s AND level = %s
            """
            count = cursor.execute(sql, [self.lft, self.rght, level])

    def __unicode__(self):
        return self.name

    def __repr__(self):
        return u'<Category: %s>' % self.name


def update_category_url(sender, instance, **kwargs):
    instance.update_url()

post_save_changed.connect(update_category_url, sender=Category, fields=['slug', 'parent'])