下面是一个使用posts_clauses 过滤器：

function search_posts_clauses($clauses, $query){
    if(!is_admin() && $query->is_main_query() && $query->is_search()){
        global $wpdb;

        // replace post_content with meta_key and meta_value:
        $searchterms = explode(" ",get_query_var("s"));
        foreach($searchterms as $searchterm){
             $from = sprintf("{$wpdb->posts}.post_content LIKE \'%%%s%%\'", $searchterm);
             $to = sprintf("{$wpdb->postmeta}.meta_key = \'cities\' AND {$wpdb->postmeta}.meta_value = \'%s\' ", $searchterm);  
             $clauses["where"] = str_replace($from, $to, $clauses["where"]);
        }

        $clauses["join"] = "INNER JOIN {$wpdb->postmeta} ON ({$wpdb->posts}.ID = {$wpdb->postmeta}.post_id) ";
    }
    return $clauses;
}
add_filter( \'posts_clauses\', \'search_posts_clauses\', 10, 2 );