yii\validators\ExistValidator::validateValue PHP Method

    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.');
        }
        $query = $this->createQuery($this->targetClass, [$this->targetAttribute => $value]);
        if (is_array($value)) {
            if (!$this->allowArray) {
                return [$this->message, []];
            }
            return $query->count("DISTINCT [[{$this->targetAttribute}]]") == count($value) ? null : [$this->message, []];
        } else {
            return $query->exists() ? null : [$this->message, []];
        }
    }