Get all stored sessions

I do something like this:
Code from model (I using IonAuth):

$this->db->insert( 'ci_sessions', array( 'logged_id' => $user->id ) );
$this->session->set_userdata($session_data);


but now, when I'm logged it's adding 'user_id' to new column - ok, but other are empty, why? (sorry, I'm not good with sessions)

I created 'Session_dummy_driver' with this method:

PHP Code:

public function write($session_id, $session_data)
        {
                $this->db->where( 'id', $session_id );
                $this->update( 'logged_id', $session_data->user_id );
        } 


But not updated 'logged_id' - where is my bad?