Basic Map/Reduce

[tmountain]

Hi guys, trying to do a basic map reduce job and running into a snag. I've tried a few basic tests, and it seems like my reduce job is always ignored. Here's an example:

> var m = function() { emit( this.s, {count : 10} ) } // this.s is a session id
> var r = function(key, values) { return {count : 20}; }
> db.tracking.mapReduce(m, r, "foo")
{
    "result" : "foo",
    "timeMillis" : 36,
    "counts" : {
        "input" : 52,
        "emit" : 52,
        "reduce" : 0,
        "output" : 52
    },
    "ok" : 1,
}
> db.foo.find()
{ "_id" : "201207_01202097a09378b506fa529bd946f356", "value" : { "count" : 10 } }
{ "_id" : "201207_01b56eac4ff502bd7da53bfab8a4ea98", "value" : { "count" : 10 } }

As you can see, all I'm trying to do here is change the value of count, but for some reason the original value is being reported.

[tmountain]

Doing some more research of my own, it seems map reduce is optimized to avoid processing single document matches. Apparently, the finalize function is preferred here.

[A. Jesse Jiryu Davis]

Your reduce function isn't called unless there are multiple documents with the same key -- in your case, unless several documents with the same 's' value are emitted by your map function.