使用组聚合时，数据类型从Int变为Float

{ \"_id\" : ObjectId(\"4ddcdc9ab4d8a3a90345508e\"), \"vehicleId\" : \"1\", \"timestamp\" : ISODate(\"2011-05-25T10:40:25.856Z\"), \"speed\" : 1 }

{ \"_id\" : ObjectId(\"4ddcdc9ab4d8a3a90345508f\"), \"vehicleId\" : \"2\", \"timestamp\" : ISODate(\"2011-05-25T10:40:26.232Z\"), \"speed\" : 2 }

val key = MongoDBObject(\"vehicleId\" -> true)
val cond = MongoDBObject.empty
val initial = MongoDBObject(\"timestamp\" -> 0)

val reduce =
  \"\"\"function(doc, prev) {
       if (doc.timestamp > prev.timestamp) {
          prev.speed = doc.speed;
          prev.timestamp = doc.timestamp;
       }
     }\"\"\"

val groupedSpeed = collection.group(key, cond, initial, reduce)

for (dbObject: DBObject <- groupedSpeed) {
  println(dbObject.toString)

{ \"vehicleId\" : \"2\" , \"timestamp\" : { \"$date\" : \"2011-05-25T10:40:49Z\"} , \"speed\" : 2.0}
{ \"vehicleId\" : \"1\" , \"timestamp\" : { \"$date\" : \"2011-05-25T10:40:49Z\"} , \"speed\" : 1.0}