turns an ebook_id into a path on PG's server(s)
            4443  -> 4/4/4/4443/

def remote_path(self):
        """ turns an ebook_id into a path on PG's server(s)
            4443  -> 4/4/4/4443/ """
        # TODO: move this property into independent object for PG
        if len(self.book_id) > 1:
            path_parts = list(self.book_id[:-1])
        else:
            path_parts = ['0']
        path_parts.append(self.book_id)
        return os.path.join(*path_parts) + '/'