problem Nested sets

[eluser]momen[/eluser]
when used Nested sets

show this error

A PHP Error was encountered
Severity: Notice

Message: Undefined property: Categories_demo_model::$db

Filename: models/Nested_sets_model.php

Line Number: 291


Fatal error: Call to a member function query() on a non-object in H:\xampp\htdocs\codeigniter\system\application\models\Nested_sets_model.php on line 291