ObjectId is incorrectly converted into string by $group

[bpiltz]

When returning a field containing an ObjectId the returned Object does not contain a valid id-String. When console.logging the direct result on the server the id is not put into ''. So it is actually a hex-number. Obviously the conversion into a string is not done correctly as the resulting string is a direct byte-to-ascii conversion which results in wired characters within the string. It can be corrected by iterating over the result and performing a byte-by-byte 'charCodeAt'-conversion (with zero-padding for numbers < 0x10). Using [email protected].