string based custom ordering in active record

[eluser]tsandesh23[/eluser]
This is my normal mysql query:

Code:

$sql = "SELECT * FROM item order by ";
if(my_condition)
{
       $sql. = "FIELD(`category`, 'Auto & Accessories', 'Couch', 'Bed',
       'Desk &     Office', 'Bike &            Scooter', 'Tools', 'Leisure',
        'Iron & Wood', 'Cabinet', 'Kitchen & Accessories', 'Refrigerator & Appliances',
        'Toys & Games', 'Chair', 'Table', 'Garden & Terrace', 'TV, HIFI & Computers',
        'General Item')";

}
else
{
     $sql .= "category asc";
}

I need it in CI in active record. I tried in following way:

Code:

if(my_condition)
            {
                $this->db->order_by("FIELD(`category`, 'Auto & Accessories', 'Couch',
                'Bed', 'Desk & Office', 'Bike & Scooter', 'Tools', 'Leisure', 'Iron &
                 Wood', 'Cabinet', 'Kitchen & Accessories', 'Refrigerator &
                 Appliances', 'Toys & Games', 'Chair', 'Table', 'Garden & Terrace',
                 'TV, HIFI & Computers', 'General Item')");

            }
            else
            {
                $this->db->order_by("category", "asc");
            }

But, I got this error::

Error Number: 1064
You have an error in your SQL syntax; check the manual that corresponds to your
MySQL server version for the right syntax to use near 'Couch'`, `'Bed'`, `'Desk` &
Office', `'Bike` & Scooter', `'Tools'`, `'Leisure'`,' at line 6


Filename: C:\xampp\htdocs\straatjutter_service\system\database\DB_driver.php

Line Number: 330
How to solve it? Actually I need my item's category sorted in above format. This can be done in normal mysql query. But, get confused in doing it in active records.