private function buildFromSubDirectory(\FilesystemIterator $directory)
    {
        /* @var $fileInfo \SplFileInfo */
        foreach ($directory as $fileInfo) {
            if ($fileInfo->isDir()) {
                $subDirectory = new \FilesystemIterator($fileInfo->getRealPath());
                $this->buildFromSubDirectory($subDirectory);
            } elseif ($fileInfo->isFile() && $fileInfo->getExtension() === $this->extension) {
                $migration = UnitOfWorkFactory::buildFromSplFileInfo($fileInfo);
                $this->add($migration);
            }
        }
    }