如何统计MONGODB中所有唯一值？ [复制]

{

    {
    "date": "2017-09-04",
    "description": "DD from my employer1",
    "amount": 1000.33
    },
    {
    "date": "2017-09-06",
    "description": "DD from my employer1",
    "amount": 1000.34
    },
    {
    "date": "2017-09-06",
    "description": "DD from my employer1",
    "amount": 1000.35
    },
    {
    "date": "2017-09-07",
    "description": "DD from employer1",
    "amount": 5000.00
    },
    {
    "date": "2017-09-08",
    "description": "DD from my employer1",
    "amount": 2000.33
    },
    {
    "date": "2017-09-09",
    "description": "DD from my employer1",
    "amount": 2000.33
    },
    {
    "date": "2017-09-10",
    "description": "DD from my employer1",
    "amount": 2000.33
    }

}

我上面有这组对象，我正在尝试计算唯一日期的数量。有 7 个带有日期字段的对象，但正如您所看到的，2017-09-06 显示了两次，这意味着总共只有 6 天。我正在尝试对我的日期进行分组，但无法让 mongo 仅计算一次日期并将出现多次的日期视为一个。

我已经尝试过这个...

{
    {
        "$group": {
            "_id": {
                "description": "$description",
                "date": "$date"   
            },
            "count": {
                "$sum": 1
            }
        }
    }
}

与骨料：

db.collection.aggregate(
    // Pipeline
    [
        {
            $group: {
                _id: "$date"
            }
        },
        {
            $group: {
               _id:1, 
               count: {$sum : 1 }
            }
        },
    ]
)