You can look this post Codeigniter - multiple database connections
but basically is like this
add this to database.php
$db['otherdb']['hostname'] = "localhost";
$db['otherdb']['username'] = "root";
$db['otherdb']['password'] = "";
$db['otherdb']['database'] = "other_database_name";
$db['otherdb']['dbdriver'] = "mysql";
$db['otherdb']['dbprefix'] = "";
$db['otherdb']['pconnect'] = TRUE;
$db['otherdb']['db_debug'] = FALSE;
$db['otherdb']['cache_on'] = FALSE;
$db['otherdb']['cachedir'] = "";
$db['otherdb']['char_set'] = "utf8";
$db['otherdb']['dbcollat'] = "utf8_general_ci";
$db['otherdb']['swap_pre'] = "";
$db['otherdb']['autoinit'] = TRUE;
$db['otherdb']['stricton'] = FALSE;
and you can call it from the model like this
function my_model_method()
{
$otherdb = $this->load->database('otherdb', TRUE); // the TRUE paramater tells CI that you'd like to return the database object.
$query = $otherdb->select('first_name, last_name')->get('person');
var_dump($query);
}