def add_parent(self, parent_id, name, mode):
        name = sanitize_unicode(name)
        parent_id = canonicalize_to_id(parent_id)
        b = TreeParentTree(key1=self.id, key2=parent_id)
        b.name = name
        b.mode = mode
        b.save()
Give this tree a parent.  Also updates the parent to know
dataset/ETHPy150Open ebroder/anygit/anygit/backends/mongodb.py/Tree.add_parent