如何在聚合管道的$ group阶段将数组转换为对象？

Question

I have a group stage in a mongodb aggregation pipeline that looks like this:

$group: {
    _id: null,
    comments: {
        $push: {
            _id: "$comments._id", 
            comment: "$comments.comment",
            creator: "$comments.creator"
        }
    }
}

The output looks like this:

{
    "_id": null,
    comments: [{
        "_id": "5c113250b54e8a5bc4412a9a",
        "comment": "This is my comment",
        "creator": [{
            "_id": "5c113250b54e8a5bb9252a7b",
            "name": "John Doe"
        }, ...]
    }]
}

Notice that creator is an array. I want it to just be an object. For some reason this isn't easy to do (it probably is, but I can't figure it out). Here is what I've tried:

$group: {
    _id: null,
    comments: {
        $push: {
            _id: "$comments._id", 
            comment: "$comments.comment",
            creator: {$arrayToObject: "$comments.creator"}
        }
    }
}

But it doesn't work. How can I convert the creator array to an object?

Answer 1

我想到了：

creator: { $arrayElemAt: [ "$comments.creator", 0 ] }