active record for like without wildcard is not work

[eluser]kreamik[/eluser]
i do this :

Code:

$this->db->like('parent_id',$parent_id,'none');

i hope this query will be :

Code:

where 'parent_id' like '$parent_id'

but i got :

Code:

where 'parent_id' like '%$parent_id%'

i see on :
codeigniter\system\database\DB_active_rec.php
method : _like

no more case for state : 'none'
i use : CI_2.1.0

is this bug?