protected function getOperator($operator) {
         /**
          * @todo change that for a factory
          */
        if(array_key_exists($operator, $this->supportedOperators)) {
            
            $operatorClass = $this->operatorNameSpace . '\\' . ($this->supportedOperators[$operator]);
            $operator = $this->getServiceLocator()->get($operatorClass);
            $operator->setDriverEscaper($this->getDriverEscaper());
            return $operator;
        }
        throw new QueryParsingException('this driver doesn\'t support ' . $operator . ' operator');
    }