wanwizard, datamapper. session wont store data in default database if use cart library

[eluser]reka[/eluser]
I using CI's sessions with database..
I have two database connection.
here is my db group in my database.php file :

Code:

$active_group = 'mahasiswa';
$active_record = TRUE;
$db['mahasiswa']['hostname'] = 'localhost';
$db['mahasiswa']['username'] = 'aplikasi';
$db['mahasiswa']['password'] = '123456';
$db['mahasiswa']['database'] = 'mahasiswa';
$db['mahasiswa']['dbdriver'] = 'mysql';
$db['mahasiswa']['dbprefix'] = '';
$db['mahasiswa']['pconnect'] = TRUE;
$db['mahasiswa']['db_debug'] = TRUE;
$db['mahasiswa']['cache_on'] = FALSE;
$db['mahasiswa']['cachedir'] = '';
$db['mahasiswa']['char_set'] = 'utf8';
$db['mahasiswa']['dbcollat'] = 'utf8_general_ci';
$db['mahasiswa']['swap_pre'] = '';
$db['mahasiswa']['autoinit'] = TRUE;
$db['mahasiswa']['stricton'] = FALSE;

$active_group = 'default';
$active_record = TRUE;
$db['default']['hostname'] = 'localhost';
$db['default']['username'] = 'aplikasi';
$db['default']['password'] = '123456';
$db['default']['database'] = 'transkrip';
$db['default']['dbdriver'] = 'mysql';
$db['default']['dbprefix'] = '';
$db['default']['pconnect'] = TRUE;
$db['default']['db_debug'] = TRUE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = '';
$db['default']['char_set'] = 'utf8';
$db['default']['dbcollat'] = 'utf8_general_ci';
$db['default']['swap_pre'] = '';
$db['default']['autoinit'] = TRUE;
$db['default']['stricton'] = FALSE;

when I submit with

Code:

$this->cart->insert($data);

my data stored to "mahasiswa.ci_sessions" instead of my default connection..
and I got an error, because there's no ci_sessions table in mahasiswa

and here's my model
mahasiswa_model.php

Code:

Class Mahasiswa_Model extends DataMapper{
     var $db_params = 'mahasiswa';
     var $table = "mahasiswa";
}