several post_parent for wp_query and create sql query…

Hello
Ia have a problem similary: http://wordpress.org/support/topic/several-post_parent-for-wp_query?replies=13

Im Trying to generate all possible post_parent second level:

add_filter('posts_where', 'add_post_parents', 1);
  function add_post_parents($where){
    $args_id_parent = array(
       'post_type' => 'produkty',
       'post_parent' => 0,
       'posts_per_page'=>-1,
       'post_status'=>'publish'
    );
    $query_id_parent=new WP_Query($args_id_parent);
    $where .= " AND (";
    while($query_id_parent->have_posts()):
      $query_id_parent -> the_post();
      $where .='post_parent = "'.get_the_ID().'" OR ';
    endwhile;
    $where=substr($where,0,-3);
    $where .= ') AND (post_status="publish")';
    return $where;
  }

On my side does not return an error to me. What I am doing wrong?
Generating “echo” such works correctly:

$args_id_parent = array(
       'post_type' => 'produkty',
       'post_parent' => 0,
       'posts_per_page'=>-1,
       'post_status'=>'publish'
    );
    $query_id_parent=new WP_Query($args_id_parent);
    $where .= " AND (";
    while($query_id_parent->have_posts()):
      $query_id_parent -> the_post();
      $where .='post_parent = "'.get_the_ID().'" OR ';
    endwhile;
    $where=substr($where,0,-3); 

    $where .= ') AND (post_status="publish")';
    echo $where;