mongodb mapreduce 聚合查询

db.detail.mapReduce(
function () {
    emit(this.userCode,1)
},
function(key, values){
    var sum = 0;
    for(var i = 0; i<values.length;i++ )
        sum += values[i]; return sum; 
},
{
       query:{"exchangeDate":{$gte:new Date("2017-01-06"),$lte:new Date("2017-01-11")},"extendName":"2%加息券","useFlag":"1"},
       out: "result",
       sort : {},
       limit : 0
})