Mongodb Mongo聚合框架，排序然后分组不起作用

我正在try 先按日期排序数据，然后在另一个字段上分组.

The question I am trying to answer is: Select the most recent distinct cid?

鉴于这些数据:

db.summary.save({"lid" : 5, "date" : 5, "cid" : 2, "circles" : [ 2 ] })
db.summary.save({"lid" : 2, "date" : 2, "cid" : 1, "circles" : [ 2 ] })
db.summary.save({"lid" : 4, "date" : 0, "cid" : 3, "circles" : [ 2 ] })
db.summary.save({"lid" : 3, "date" : 3, "cid" : 2, "circles" : [ 2 ] })
db.summary.save({"lid" : 1, "date" : 1, "cid" : 1, "circles" : [ 2 ] })

db.summary.aggregate( {$match :{circles: 2}, $sort: {date: -1}, $group: {_id: '$cid'}} )

我先在圆圈上做一场比赛，

我得到的结果是:

{
    "result" : [
        {
            "_id" : 3
        },
        {
            "_id" : 1
        },
        {
            "_id" : 2
        }
    ],
    "ok" : 1
}

Here is my analysis:

在按日期进行匹配或排序之前，数据为:

"lid" : 5, "date" : 5, "cid" : 2
"lid" : 2, "date" : 2, "cid" : 1
"lid" : 4, "date" : 0, "cid" : 3
"lid" : 3, "date" : 3, "cid" : 2
"lid" : 1, "date" : 1, "cid" : 1

按日期排序后，数据集将为:

"lid" : 5, "date" : 5, "cid" : 2
"lid" : 3, "date" : 3, "cid" : 2
"lid" : 2, "date" : 2, "cid" : 1 
"lid" : 1, "date" : 1, "cid" : 1
"lid" : 4, "date" : 0, "cid" : 3

所以分组后，我预期的结果是:

{
    "result" : [
        {
            "_id" : 2
        },
        {
            "_id" : 1
        },
        {
            "_id" : 3
        }
    ],
    "ok" : 1
}

什么问题解决了我的问题？

为什么当前的查询对我不起作用？