why pre_get_posts works fine in post type archive, but not in search result list?

Question

I have sorted my WooCommerce product list (Training events) with a pre_get_posts Filter (as shown below), to sort by Date via custom field "startdate2". This works fine for the WooCommerce product list, but does not affect the sorting of a search result list, displayed via the same template (archive-product.php).

The product posts in search result are always in wrong order. I tried out a lot but nothing works and I can't figure out why. Can it be that the pre_get_post filter does NOT work for search results? Or is it probably necessary to write a second pre_get_post filter only for the search result list?

I also tried out an extra query in the template only for "is_search()" - but does not work too.

// Sort Products by startdate2
function my_pre_get_posts( $query ) {
    // do not modify queries in the admin
    if ( is_admin() ) {
        return $query;
    }

    // only modify queries for 'product' post type
    if( isset($query->query_vars['post_type']) && $query->query_vars['post_type'] == 'product' ) {

        $query->set('orderby', 'meta_value');
        $query->set('meta_type', 'DATE');
        $query->set('meta_key', 'startdate2');
        $query->set('order', 'ASC');


    }
    // return
    return $query;
}
add_action('pre_get_posts', 'my_pre_get_posts');

// alternative: Extra query in template:
$args = array(
    'post_type'   => 'product',
    'orderby' => 'meta_value_num',
    'meta_key'=> 'startdate2',
    'order'   => 'ASC'
);

// the query
$the_query = new WP_Query( $args ); 

if ( $the_query->have_posts() ) : 

    while ( $the_query->have_posts() ) : $the_query->the_post(); 
         wc_get_template_part( 'content', 'product' ); 
    endwhile; 

    wp_reset_postdata(); 

Answer 1

If searching via Wordpress ?s=keyword (not WooComm's Product Search), post_type isn't set or used, so the following would return false:

if( isset($query->query_vars['post_type']) && $query->query_vars['post_type'] == 'product' ) {

You can explore what $query->query_vars contains on your Products page vs your Products Search page by just printing it and killing the page:

function my_pre_get_posts( $query ) {
  echo "<pre>".print_r($query->query_vars,true)."</pre>";
  die;
  //...

---

If you are using WooComm's search (and the query is like ?s=keyword&post_type=product) and WooComm does have post_type set, try firing your action with a later priority:

add_action('pre_get_posts', 'my_pre_get_posts',999);