Quoting https://stackoverflow.com/a/6453755/1150636
Both document structure and collection structure in MongoDB based on JSON principles. JSON is a set of key/value pairs (in particular fieldName/fieldValue for document and index/document for collection). From this point of view it doesn't seem that you can rely on order at all.
This means that there is no particular order that mongodb is required to return the fields of one record. You need to be ready to parse the results in whichever order they may be returned.