question in the dbforge class

[eluser]Mr-H[/eluser]
hi every body
i want to creat a table in the phpmyadmin with the dbforge class using UNIQUE key.
but in the user guide nothing pointed to that, it said that i can creat a primary key using:

Code:

$this->dbforge->add_key('id',true);

and creat key "what kind of key? i dont know" by puting this;

Code:

$this->dbforge->add_key('test');

so when i run the code i got the id as primary, and test as index" by default i think".
so my question is how can i creat the test field as Unique key.
thanks for reading