public static function activate()
 {
     global $wpdb;
     global $series_table_name;
     global $posts_series_table_name;
     $series_db = new Easy_Series_Db($wpdb, $series_table_name);
     if (!$series_db->has_table()) {
         $series_db->create_table();
     }
     $posts_series_db = new Easy_Series_Posts_Db($wpdb, $posts_series_table_name);
     if (!$posts_series_db->has_table()) {
         $posts_series_db->create_table();
     }
 }