cdbcriteria check if relation is empty

I would like to show only books that dont have feedback.

$criteria = new CDbCriteria;

        $criteria->with = array('feedbacks');


        $criteria->compare('feedbacks.reserva', null);


$criteria->compare('feedbacks', null);


try something like

$criteria->compare('', null);

where "id" should be PK column from feedbacks table

or (I think better) do not join with feedbacks but instead:

$criteria->addCondition( 'NOT EXISTS (SELECT id FROM feedback WHERE feedback.id_book =' );

Thanks redguy, second aproach works .

Best way is to set up relation condition

$criteria->addCondition(‘ IS NULL’);

nice :)