Include posts from Co-Authors Plus Guest Authors in Wordpress' search results

search-posts-by-guest-authors.php

<?php defined('ABSPATH') or die();

/**
 * Include posts from guest authors in the search results.
 * @author jakebellacera
 */
add_filter('posts_clauses', 'search_posts_clauses', 10, 2);
function search_posts_clauses($clauses, $query) {
  if (!$query->is_main_query && !$query->is_search && !$query->is_admin)
    return $clauses;

  global $wpdb;

  $clauses['join'] .= <<<SQL
INNER JOIN {$wpdb->term_relationships} ON {$wpdb->posts}.ID = {$wpdb->term_relationships}.object_id
INNER JOIN {$wpdb->term_taxonomy} USING (term_taxonomy_id)
INNER JOIN {$wpdb->terms} USING (term_id)
SQL;

  $clauses['where'] .= $wpdb->prepare(
    " OR ({$wpdb->term_taxonomy}.taxonomy = 'author' AND {$wpdb->term_taxonomy}.description LIKE '%%%s%%' AND {$wpdb->posts}.post_type = 'post')",
    sanitize_text_field($query->query['s'])
  );

  return $clauses;
}