/**
  * @return string
  */
 public function getDQLCondition()
 {
     $string = $this->value->getKey() . ' ';
     $string .= $this->value->getOperator() . ' ';
     if ($this->value->getOperator() === 'BETWEEN') {
         $keys = [];
         foreach ($this->value->getValue() as $index => $v) {
             $keys[] = $this->value->getReferenceKey($index);
         }
         $string .= implode(' AND ', $keys);
     } elseif ($this->value->getOperator() === 'IN') {
         $string .= '(' . $this->value->getReferenceKey() . ')';
     } elseif ($this->value->getOperator() === 'IS') {
         $string .= $this->value->getValue();
     } elseif ($this->value->getOperator() === 'INSTANCE OF') {
         $string .= $this->value->getValue();
     } else {
         $string .= $this->value->getReferenceKey();
     }
     return $string;
 }