在mongodb中获取周开始日期

Question

我需要从星期数中获取星期开始日期，在下面的查询中，我只能获取特定日期的星期数（ModifiedOn）。我如何使用mongo查询获取Weekstart日期星期几。

需要星期开始日期而不是星期编号

db.userstoryaggregateview.aggregate([
    {
        "$group": {
            "_id": {
                "ClientUId": "$ClientUId",
                "DeliveryConstructUId": "$DeliveryConstructUId",
                "ReleaseUId": "$ReleaseUId",
                "IterationUId": "$IterationUId",
                "TeamAreaUId": "$TeamAreaUId",
                "Week" : { $week : "$ModifiedOn" }
            },
            "UserStory-SPEstimatedTotalWeekly": {
                    "$sum": {
                        "$cond": [
                            {
                                "$eq": [
                                    "$UserStory-StoryPointEstimatedTotal",
                                    ""
                                ]
                            },
                            NumberInt("0"),
                            { $toDouble: "$UserStory-StoryPointEstimatedTotal" }
                        ]
                    }
            }
        }
    },
    {
        $project: {
            "_id": NumberInt("0"), 
            "ClientUId": "$_id.ClientUId", "DeliveryConstructUId": "$_id.DeliveryConstructUId",
            "ReleaseUId": "$_id.ReleaseUId", "IterationUId": "$_id.IterationUId", "TeamAreaUId": "$_id.TeamAreaUId", 
            "Value": "$UserStory-SPEstimatedTotalWeekly",
            "Week":"$_id.Week",
              weekStart: { $dateToString: { format: "%Y-%m-%d", date: { // convert date
             $subtract: ["$ModifiedOn", {$multiply: [ {$subtract:[{$isoDayOfWeek: "$ModifiedOn"},1]}, 86400000]}] 
           }}},

        }
    }], { allowDiskUse: true, cursor:{ batchSize: 101} });