Model_Books_Users

class Model_Books_User extends \Orm\Model {

    protected static $_properties = array(
        'user_id',
        'book_id',
    );
    protected static $_table_name = 'books_users';

    protected static $_many_many = array(
        'users' => array(
            'key_from' => 'id',
            'key_through_from' => 'book_id',
            'table_through' => 'books_users',
            'key_through_to' => 'user_id',
            'model_to' => 'Model_User',
            'key_to' => 'id',
            'cascade_save' => true,
            'cascade_delete' => false,
        )
    );
}