在MongoDB的另一个字段上分组的文档中，字段中字符串实例的计数？

我有一个特定的用例，我正试图找到一种方法在一个聚合管道中完成它，最好不需要硬编码任何数据值。我希望基于一个属性对文档进行分组，并查看文档中特定字段的值计数

示例数据：

{
    "flightNum": "DL1002",
    "status": "On time",
    "date": 20191001
},
{
    "flightNum": "DL1002",
    "status": "Delayed",
    "date": 20191002
},
{
    "flightNum": "DL1002",
    "status": "On time",
    "date": 20191003
},
{
    "flightNum": "DL1002",
    "status": "Cancelled",
    "date": 20191004
},
{
    "flightNum": "DL952",
    "status": "On time",
    "date": 20191003
},
{
    "flightNum": "DL952",
    "status": "On time",
    "date": 20191004
}

{
    "flightNum": "DL1002",
    "numOnTime": 2,
    "numCancelled": 1,
    "numDelayed": 1
},
{
    "flightNum": "DL952",
    "numOnTime": 2
}

我想要一个聚合管道，可以告诉我，对于每个航班（按flightNum分组），有多少航班“准时”、“延误”或“取消”

期望的响应：

{
    "flightNum": "DL1002",
    "status": "On time",
    "date": 20191001
},
{
    "flightNum": "DL1002",
    "status": "Delayed",
    "date": 20191002
},
{
    "flightNum": "DL1002",
    "status": "On time",
    "date": 20191003
},
{
    "flightNum": "DL1002",
    "status": "Cancelled",
    "date": 20191004
},
{
    "flightNum": "DL952",
    "status": "On time",
    "date": 20191003
},
{
    "flightNum": "DL952",
    "status": "On time",
    "date": 20191004
}

{
    "flightNum": "DL1002",
    "numOnTime": 2,
    "numCancelled": 1,
    "numDelayed": 1
},
{
    "flightNum": "DL952",
    "numOnTime": 2
}

---

字段的命名并不重要，因为它们存在于一个文档中。我发现我可以用操作符来实现这一点，但这需要我硬编码“status”字段的预期值。对于这个任意的例子，没有太多的值，但是如果添加了另一个状态，我希望不必更新查询。既然Mongo有这么多漂亮的把戏，我觉得有可能实现这一点

您可以尝试以下查询：

db.collection.aggregate([
    /** Group all docs based on flightNum & status & count no.of occurances */
    {
        $group: {
            _id: {
                flightNum: "$flightNum",
                status: "$status"
            },
            count: {
                $sum: 1
            }
        }
    },
    /** Group on flightNum & push an objects with key as status & value as count */
    {
        $group: {
            _id: "$_id.flightNum",
            status: {
                $push: {
                    k: "$_id.status",
                    v: "$count"
                }
            }
        }
    },
    /** Recreate status field as an object of key:value pairs from an array of objects */
    {
        $addFields: {
            status: {
                $arrayToObject: "$status"
            }
        }
    },
    /** add flightNum inside status object */
    {
        $addFields: {
            "status.flightNum": "$_id"
        }
    },
    /** Replace status field as new root for each doc in coll */
    {
        $replaceRoot: {
            newRoot: "$status"
        }
    }
])

---

测试：

在whoami的@RyanGunner中添加了一些糖：糟糕，我忽略了必需的o/p&没有将状态字段名称转换为必需的o/p，谢谢！！哎呀，我意识到我增加了一个额外的航班号，这不是我的本意，所以我的预期结果与我的数据不符。我修好了。尽管如此，whoami的管道还是起了作用。谢谢非常圆滑！谢谢