code: protected function validateValue($value)
      {
          if ($this->targetClass === null) {
              throw new InvalidConfigException('The "targetClass" property must be set.');
          }
          if (!is_string($this->targetAttribute)) {
              throw new InvalidConfigException('The "targetAttribute" property must be configured as a string.');
          }
  
          if (is_array($value) && !$this->allowArray) {
              return [$this->message, []];
          }
  
          $query = $this->createQuery($this->targetClass, [$this->targetAttribute => $value]);
  
          return $this->valueExists($this->targetClass, $query, $value) ? null : [$this->message, []];
      }
docs: {@inheritdoc}