public function setAggregator(QueryAggregatorInterface $aggregator = null)
    {
        // Use the default aggregator if none was set
        if (!$aggregator) {
            if (!self::$defaultAggregator) {
                self::$defaultAggregator = new PhpAggregator();
            }
            $aggregator = self::$defaultAggregator;
        }

        $this->aggregator = $aggregator;

        return $this;
    }