javascript - MongoDB $project 嵌入文档到根级别

Question

使用聚合管道，我正在尝试将嵌入文档投影到根级别不单独投影每个字段。

例如，我想将 name 从这个集合中投影到根级别:

[
    {
        _id: "1",
        name: {
            firstName: "John",
            lastname: "Peters"
        }
    },
    {
        _id: "2",
        name: {
            firstName: "Mary",
            lastname: "Jones"
        }
    }
]

这就是我要找的:

[
    {
        firstName: "John",
        lastname: "Peters"
    },
    {
        firstName: "Mary",
        lastname: "Jones"
    }
]

有没有办法在不单独投影每个字段的情况下做到这一点？我不想这样做:

db.collection.aggregate(
    [
        {
            $project : {
                "_id" : 0,
                "firstName" : "$name.firstName",
                "lastName" : "$name.lastName"
            }
        }
    ]

Answer 1

MongoDB 3.4 在聚合管道中有一个新阶段 - $replaceRoot，它完全按照要求进行。

https://docs.mongodb.com/manual/reference/operator/aggregation/replaceRoot/