如果你指的是fields的顺序有变化，那是因为JSON本身定义Object是一个fields的集合，是无序的。但是MongoDB主动地维护属性的顺序。
MongoDB preserves the order of the document fields following write operations except for the following cases:
- The _id field is always the first field in the document.
- Updates that include renaming of field names may result in the reordering of fields in the document.
Changed in version 2.6: Starting in version 2.6, MongoDB actively attempts to preserve the field order in a document. Before version 2.6, MongoDB did not actively preserve the order of the fields in a document.
那属性的顺序为什么会变呢，这是因为各个语言中对集合的实现不一样。比如python中的dict不维护顺序，所以他传到driver中的时候顺序已经变了，如果希望保持顺序，driver都提供了有序集合，比如python里的 SON.