Subject:	Mongo Framework Aggregation help

Hello Dev Support,

I have a dumb little script that I can seem to get working.

I have a collection of Java Garbage Collection documents, I want to search and 
get the last free percentage of memory for each port/host.

I'm sure I'm doing something stupid, but the error message doesn't really help 
me ….

Regards,

Craig Curtin
(201)557-1132
BAML
============
MongoDB shell version: 2.4.1


script:
// specify which database to use
use tmp
// call the aggregator on all documents in the collection
db.RSgc.ensureIndex( { utime: 1 } )
db.RSgc.aggregate(
[
        // now group the documents how we want to see them
        { $group : {
                        _id : { myhost : "$host",
                                myport : "$port",
                                //freeMem : { $last : "$postFreePct" },
                        }
                 }
        }
]
)

ouput:


                {
                        "_id" : {
                                "myhost" : "mrsp3.mrisk.bankofamerica.com",
                                "myport" : 4311
                        }
                },
                {
                        "_id" : {
                                "myhost" : "mrsp2.mrisk.bankofamerica.com",
                                "myport" : 4306
                        }
                }
        ],
        "ok" : 1

... all is good and well, now I want the "Last Free memory Percentage", so I 
uncomment above and get:
Error message: 
-sh-3.2$ ./mongo <    mongJira.20130503
MongoDB shell version: 2.4.1
Fri May  3 13:42:46.186 versionArrayTest passed
connecting to: tmp
Fri May  3 13:42:46.221 creating new connection to:127.0.0.1:27017
Fri May  3 13:42:46.221 BackgroundJob starting: ConnectBG
Fri May  3 13:42:46.221 connected connection!
switched to db tmp
Error: Printing Stack Trace
    at printStackTrace (src/mongo/shell/utils.js:37:7)
    at DBCollection.aggregate (src/mongo/shell/collection.js:897:1)
    at (shell):1:9
Fri May  3 13:42:46.297 JavaScript execution failed: aggregate failed: {
        "code" : 15999,
        "ok" : 0,
        "errmsg" : "exception: invalid operator '$last'"
} at src/mongo/shell/collection.js:L898
bye



this are few sample documents
 {
   "_id": ObjectId("5182d7a1b81bd203f42f99e9"),
   "upTime": 117.99439277778,
   "gcMode": "GC(Partial)",
   "postFreePct": 46.314380787037,
   "postMem": NumberInt(29686),
   "freeMemStr": "012345",
   "host": "mrsp6.mrisk.bankofamerica.com",
   "recvMem": NumberInt(3),
   "time": ISODate("2013-05-02T21:15:47.0Z"),
   "maxHeapSize": NumberInt(55296),
   "port": NumberInt(4329),
   "utime": NumberLong(1367529347)
}	
 {
   "_id": ObjectId("5182d7a1b81bd203f42f99e8"),
   "upTime": 117.99413083333,
   "gcMode": "GC(Partial)",
   "postFreePct": 46.355975115741,
   "postMem": NumberInt(29663),
   "freeMemStr": "012345",
   "host": "mrsp6.mrisk.bankofamerica.com",
   "recvMem": NumberInt(9),
   "time": ISODate("2013-05-02T21:15:46.0Z"),
   "maxHeapSize": NumberInt(55296),
   "port": NumberInt(4329),
   "utime": NumberLong(1367529346)
}