public function getClassVersion($className)
    {
        if (isset($this->classVersions[$className])) {
            return $this->classVersions[$className];
        }

        $class = $this->getReflectionClass($className);

        $versionAnnotation = $this->reader->getClassAnnotation(
            $class,
            'Evispa\ObjectMigration\Annotations\Version'
        );

        if (null === $versionAnnotation) {
            throw new Exception\NotVersionedException($className);
        }

        $version = $versionAnnotation->version;

        $this->classVersions[$className] = $version;

        return $version;
    }