Mongo Map-Reduce查询

Question

查询：-

db.items.mapReduce(function(){emit(this.name,this.price);},function(key,value){return Array.sum(value);},{out:"map_reduce_example"})

输出：-

{
    "result" : "map_reduce_example",
    "timeMillis" : 293,
    "counts" : {
        "input" : 4,
        "emit" : 4,
        "reduce" : 0,
        "output" : 4
    },
    "ok" : 1
}

为什么不打印价格总和？