Doctrine\ODM\MongoDB\Query\Builder::getDiscriminatorValues PHP Method

    private function getDiscriminatorValues($classNames)
    {
        $discriminatorValues = array();
        $collections = array();
        foreach ($classNames as $className) {
            $class = $this->dm->getClassMetadata($className);
            $discriminatorValues[] = $class->discriminatorValue;
            $key = $class->getDatabase() . '.' . $class->getCollection();
            $collections[$key] = $key;
        }
        if (count($collections) > 1) {
            throw new \InvalidArgumentException('Documents involved are not all mapped to the same database collection.');
        }
        return $discriminatorValues;
    }