javascript - Mongoose 按数组最后一个对象中的属性分组

Question

我有以下数据:

[
    {
        name: "Alert 1",
        followusp: [
            {
                status: "new",
                date: "now"
            },
            {
                status: "do_it",
                date: "now"
            }
        ]
    },
    {
        name: "Alert 2",
        followusp: [
            {
                status: "new",
                date: "now"
            }
        ]
    },
    {
        name: "Alert 3",
        followusp: [
            {
                status: "new",
                date: "now"
            },
            {
                status: "engaged",
                date: "now"
            },
            {
                status: "canceled",
                date: "now"
            }
        ]
    },
    {
        name: "Alert 4",
        followusp: [
            {
                status: "new",
                date: "now"
            },
            {
                status: "canceled",
                date: "now"
            }
        ]
    }
]

我想根据followups 数组中last 对象的状态对计数 进行分组。
所以对于给出的例子，结果应该是:

[
    {
        _id: "do_it",
        count: 1
    },
    {
        _id: "new",
        count: 1
    },
    {
        _id: "canceled",
        count: 2
    }
]

目前，我坚持使用以下解决方案(它不起作用):

query.push({ $group: { _id: "$followups.status", count: { $sum: 1 } } });
return dao.aggregate(query);

Answer 1

您可以使用 $let和 $arrayElemAt传递代表最后一个数组元素的 -1 来构建您的分组键:

db.collection.aggregate([
    {
        $group: {
            _id: {
                $let: {
                    vars: { last: { $arrayElemAt: [ "$followsup", -1 ] } },
                    in: "$$last.status"
                }
            },
            count: { $sum: 1 }
        }
    }
])

Mongo Playground