WordPress: add custom taxonomies to search query.

add_custom_taxonomies_to_search_query.php

/**
 * Filter: extend search query to look for custom taxonomies.
 */
add_filter('posts_where', function ($where) {
    global $wpdb;

    if (is_search()) {
        $where .= " OR (t.name LIKE '%".get_search_query() . "%' AND {$wpdb->posts}.post_status = 'publish') ";
    }

    return $where;
});

/**
 * Filter: add post_terms tables to search query.
 */
add_filter('posts_join', function ($join) {
    global $wpdb;

    if (is_search()) {
        $join .= "LEFT JOIN {$wpdb->term_relationships} tr ON {$wpdb->posts}.ID = tr.object_id
            LEFT JOIN {$wpdb->term_taxonomy} tt ON tt.term_taxonomy_id = tr.term_taxonomy_id
            LEFT JOIN {$wpdb->terms} t ON t.term_id = tt.term_id";
    }

    return $join;
});

/**
 * Filter: add 'group by' to query to prevent duplicate post results.
 */
add_filter('posts_groupby', function ($groupby) {
    global $wpdb;

    $groupby_id = "{$wpdb->posts}.ID";
    if (!is_search() || strpos($groupby, $groupby_id) !== false){
        return $groupby;
    }

    if (!strlen(trim($groupby))){
        return $groupby_id;
    }

    return $groupby . ", " . $groupby_id;
});