/**
  * Returns an singleton instance of this class
  * @return
  */
 public static function getInstance()
 {
     if (self::$instance == null) {
         self::$instance = new DeviceCounterHistoryMapper();
     }
     return self::$instance;
 }
 /**
  * Initializes DB mappers
  *
  * @return
  */
 function __construct()
 {
     $this->mapper = DeviceCounterHistoryMapper::getInstance();
 }