I have a collection with a following data:
{ \"_id\" : ObjectId(\"4e3951905e746b3805000000\"), \"m\" : \"hello\", \"r\" : [{ \"_id\" : ObjectId(\"4e
dbh.users.update({"_id": ObjectId("4e39519d5e746bc00f000000")}, {"$unset":{"r":1}},False,False)
Try using unset
Reference: MongoDB : Update Modifier semantics of "$unset"