Crazy with callbacks

I show you the code of the other case. I seeing that this solved is_unique.

Model:

PHP Code:

public function username_check($username) {
            $account = $this->load->database('account', TRUE);
            $account->where('login',$username);
            $query = $account->get('account');
            if ($query->num_rows() > 0){
                echo $query->num_rows();
                $this->form_validation->set_message('username_check_exist', 'L´username è già utilizzato.');
                return FALSE;
            }else{
                return TRUE;
            }
        } 


Controller:

PHP Code:

public function username_check_exist($username) {
        $this->auth->username_check($username);
    }
public function register() {
        $rules = array(
                array(
                    'field'=>'username',
                    'label'=>'Account ID',
                    'rules'=>'trim|required|min_length[6]|max_length[16]|callback_username_check_exist'
                ),
ecc.... 


In this case I can try to stamp the $query->num_rows() and the result is 1 in the page... but It does not show the error code.

And if I fill out all the fields also launches the query.