mongo group

http://www.tuicool.com/articles/nMv6fi 去重复
http://www.runoob.com/mongodb/mongodb-aggregate.html
http://my.oschina.net/antianlu/blog/345972


http://blog.csdn.net/caiknife/article/details/8778910
db.runCommand({"group": {
    "ns": "stocks",
    "key": {"day": true},
    "initial": {"time": "0"},
    "$reduce": function(doc, prev) {
        if (doc.time > prev.time) {
            prev.price = doc.price;
            prev.time = doc.time;
        }
    }
}});