// MySQLDir :: { id :: Number, name :: String, lft :: Number, rgt :: Number }
// Directory :: { id :: Number, name :: String, children :: Array Directory }
// unflatten :: Array MySQLDir -> Array Directory
const unflatten = directories => {
  if (directories.length === 0) { return []; }
  const head = directories[0];
  const children = directories.filter(dir => dir.lft > head.lft && dir.rgt < head.rgt);
  const remainder = directories.filter(dir => dir.lft > head.rgt);
  return [{id: head.id, name: head.name, children: unflatten(children)}]
  .concat(unflatten(remainder));
};