I solved filtering the query by adding the join on the postmeta table and changing the where clause.
tips on filtering the WHERE clause (often require regular expression search&replace) are here on codex:
add_filter( 'posts_join', 'segnalazioni_search_join' );
function segnalazioni_search_join ( $join ) {
global $pagenow, $wpdb;
// I want the filter only when performing a search on edit page of Custom Post Type named "segnalazioni".
if ( is_admin() && 'edit.php' === $pagenow && 'segnalazioni' === $_GET['post_type'] && ! empty( $_GET['s'] ) ) {
$join .= 'LEFT JOIN ' . $wpdb->postmeta . ' ON ' . $wpdb->posts . '.ID = ' . $wpdb->postmeta . '.post_id ';
}
return $join;
}
add_filter( 'posts_where', 'segnalazioni_search_where' );
function segnalazioni_search_where( $where ) {
global $pagenow, $wpdb;
// I want the filter only when performing a search on edit page of Custom Post Type named "segnalazioni".
if ( is_admin() && 'edit.php' === $pagenow && 'segnalazioni' === $_GET['post_type'] && ! empty( $_GET['s'] ) ) {
$where = preg_replace(
"/\(\s*" . $wpdb->posts . ".post_title\s+LIKE\s*(\'[^\']+\')\s*\)/",
"(" . $wpdb->posts . ".post_title LIKE $1) OR (" . $wpdb->postmeta . ".meta_value LIKE $1)", $where );
$where.= " GROUP BY {$wpdb->posts}.id"; // Solves duplicated results
}
return $where;
}