Writing custom session data to database

[eluser]Unknown[/eluser]
I am trying to write custom session data to a database.

I have enabled database session logging in the database and setup the ci_session table in my db...

$config['sess_use_database'] = TRUE;

--

I am attempting to add custom data using an array, as described in the user guide...

Code:

$newdata = array(
                           'trip_id'  => $trip_id,
                           'quanity'     => '1'
                        );

        $this->session->set_userdata($newdata);

...but only the default session data in being written to the database.

Any ideas or steps I am missing?

Thanks,