rtb@mon-raw> map = function () { 
...     emit(this.geo.country,{count:1}); 
... } 
function () { 
    emit(this.geo.country, {count:1}); 
} 

rtb@mon-raw> reduce = function (key, values) { 
...     var result = {count:0}; 
...     values.forEach(function(doc) { 
...        result.count += doc.count; 
...     }); 
...   return result; 
... } 
function (key, values) { 
    var result = {count:0}; 
    values.forEach(function (doc) {result.count += doc.count;}); 
    return result; 
} 

rtb@mon-raw> db.request.mapReduce(map, 
...                      reduce, 
...                      {query:{"created.simple": 
{$gte:"20120207192900",$lt:"20120207193000"}}, 
...                       out:"test_coll"}) 
{ 
        "result" : "test_coll", 
        "timeMillis" : 12494, 
        "counts" : { 
                "input" : 4229, 
                "emit" : 4229, 
                "reduce" : 133, 
                "output" : 62 
        }, 
        "ok" : 1, 
} 

rtb@mon-raw> db.request.mapReduce(map, 
...                      reduce, 
...                      {query:{"created.simple": 
{$gte:"20120207192900",$lt:"20120207193000"}}, 
...                       out:{inline:1}}) 
Thu Feb 16 16:43:46 uncaught exception: map reduce failed:{ 
        "assertion" : "assertion db/commands/mr.cpp:400", 
        "errmsg" : "db assertion failure", 
        "ok" : 0 
}