小编Hit*_*osu的帖子

假设我有一个包含这样文档的集合(只是简化示例,但它应该显示该方案):

> db.data.find()
{ "_id" : ObjectId("4e9c1f27aa3dd60ee98282cf"), "type" : "A", "value" : 11 }
{ "_id" : ObjectId("4e9c1f33aa3dd60ee98282d0"), "type" : "A", "value" : 58 }
{ "_id" : ObjectId("4e9c1f40aa3dd60ee98282d1"), "type" : "B", "value" : 37 }
{ "_id" : ObjectId("4e9c1f50aa3dd60ee98282d2"), "type" : "B", "value" : 1 }
{ "_id" : ObjectId("4e9c1f56aa3dd60ee98282d3"), "type" : "A", "value" : 85 }
{ "_id" : ObjectId("4e9c1f5daa3dd60ee98282d4"), "type" : "B", "value" : 12 }

现在我需要收集一些关于该集合的统计数据.例如:

db.data.mapReduce(function(){
        emit(this.type,this.value);
     },function(key,values){
        var total = 0;
        for(i in values) …