This method satisfies the Enumerating Indexes specification's requirement
that index fields be made accessible under their original names. It may
also be used to access fields that do not have a helper method.
또한 보기:
http://php.net/arrayaccess.offsetget
또한 보기:
https://github.com/mongodb/specifications/blob/master/source/enumerate-indexes.rst#getting-full-index-information