public function delete_by_id($id)
 {
     parent::delete_by_id($id);
     $cache_files = glob($this->get_cache_dir_name() . '/' . $id . '.*');
     //echo 'print_r($cache_files): ' . "\n";
     //print_r($cache_files);
     foreach ($cache_files as $cache_file) {
         unlink($cache_file);
     }
 }
 public function delete_by_id(Database_Table $table, $id)
 {
     $table->delete_by_id($id);
 }