You’re my hero 🙂 Thank you.
Final code:
add_action( 'pre_get_posts', 'book_custom_orderby' );
function book_custom_orderby( $query ) {
if ( ! is_admin() )
return;
if (empty($query->query_vars['post_type']) || $query->query_vars['post_type'] != 'book') {
return;
}
if (empty($_GET['orderby'])) {
$query->set( 'meta_key', 'order-no' );
$query->set( 'orderby', 'meta_value_num' );
$query->set( 'order', 'ASC' );
}
}