MongoDB聚合排序不起作用

我在将排序应用于聚合分组时遇到问题.我的原始数据如下所示:

    {
            "_id" : ObjectId("52deab2fe4b0a491abb54108"),
            "type" : "build",
            "time" : ISODate("2014-01-21T17:15:27.471Z"),
            "data" : {
                    "buildNumber" : 43,
                    "buildDuration" : 997308,
                    "buildProjectName" : "TestABC",
                    "buildResult" : "SUCCESS"
            }
    }

我想先按buildProjectName排序，然后按日期排序.以下是我的疑问:

db.builds.aggregate([
    { $group: { 
        _id: { 
            month: { $month: "$time" },
            day: { $dayOfYear: "$time" },
            year: { $year: "$time" }, 
            buildProjectName: "$data.buildProjectName", 
        },
        buildDuration: { $avg: "$data.buildDuration" } 
    } },
    { $sort: {buildProjectName: 1, year: 1, month: 1, day: 1} }
])

我try 过切换排序顺序(例如:buildProjectName、day、month、year)，但如果日期不符合顺序，我总是得到相同的结果:

{
        "result" : [
                {
                        "_id" : {
                                "month" : 1,
                                "day" : 20,
                                "year" : 2014,
                                "buildProjectName" : "TestABC"
                        },
                        "buildDuration" : 1170723.5
                },
                {
                        "_id" : {
                                "month" : 1,
                                "day" : 21,
                                "year" : 2014,
                                "buildProjectName" : "TestABC"
                        },
                        "buildDuration" : 2284863.3333333335
                },
                {
                        "_id" : {
                                "month" : 1,
                                "day" : 17,
                                "year" : 2014,
                                "buildProjectName" : "TestABC"
                        },
                        "buildDuration" : 2234662
                }
        ],
        "ok" : 1
}