请问怎么实现无限级目录?

[GoogleCodeExporter]

V2.0版本，后台的目录无法实现无限级目录方法，加一些代码�
��无法写下去了。
class Category(db.Model):
    parent_cat=db.SelfReferenceProperty()

    def children(self):
        key=self.key()
        return [c for c in Category.all().filter('parent_cat =',self)]

Original issue reported on code.google.com by jxlifar...@gmail.com on 26 Oct 2013 at 4:13