MongoDB-使用聚合管道进行评论投票/降票 [英] MongoDB - Comment Upvoting/Downvoting with Aggregation Pipeline

问题描述

我正在尝试对评论实施upvote/downvote机制(类似于reddit上的upvoting/downvoting机制).我有一个名为commentReputation的单独集合，其中的文档如下所示:

I'm trying to implement an upvote/downvote mechanism for comments (similar to the upvoting/downvoting mechanism found on reddit). I have a separate collection called commentReputation and the documents inside can look like this:

{
    "_id" : ObjectId("5e5acb6d6034a879655c8819"),
    "commentId" : ObjectId("5e5983102328a83d1a4b541f"),
    "creationDate" : ISODate("2020-02-29T20:37:01.509Z"),
    "upvotes" : [
        ObjectId("5e5983102328a83d1a4b53e7"),
        ObjectId("5e5983102328a83d1a4b53e4")
    ],
    "downvotes" : [
       ObjectId("5e5983102328a83d1a4b53e5")
    ]
}

简而言之:每个评论最终都会拥有它自己的CommentReputation文档(CommentReputation文档应在有人投票赞成/反对投票时立即创建)

In short: every comment will eventually have it's own CommentReputation document (the CommentReputation document should be created as soon as someone upvotes/downvotes a comment)

有2种情况:

1. 该集合为空，这意味着我需要使用给定的commentId x创建我的第一个CommentReputation文档.在项目的其他部分，我将$setOnInsert与{ upsert: true }结合使用，但(从文档中看)似乎聚合管道目前不支持$setOnInsert.有没有其他方法可以解决这个问题?

1. The collection is empty meaning that I need to create my very first CommentReputation document with a given commentId x. In some other part of the project I was using $setOnInsert with { upsert: true } but it seems (looking at the documentation) that the aggregation pipeline does not support $setOnInsert as for now. Is there another way to deal with this problem?

文档在那里，应该进行实际的表决.

The document is there and the actuall upvoting should occur.

a)upvotes和downvotes数组均不包含试图进行投票的userId，因此无需任何进一步操作即可将其添加到upvotes数组中

a) Both upvotes and downvotes arrays do not contain the userId that is trying to upvote thus it gets added to the upvotes array without any further actions

b)upvotes数组包含试图对注释进行投票的userId，因此应从upvotes数组中删除userId. (用户已经对此评论进行了投票，并再次点击了取消投票"按钮，取消了该投票)

b) The upvotes array contains the userId that is trying to upvote the comment as a result the userId should be REMOVED from the upvotes array. (the user already had this comment upvoted and clicked a second time the upvote button which cancels out the upvote)

c)downvotes数组包含userId.在这种情况下，应从downvotes中删除userId并将其添加到upvotes

c) The downvotes array contains the userId. In this case the userId should be removed from downvotes and added to upvotes

我正在尝试使用updateOne方法和聚集管道来完成上述逻辑，但是我不确定这是否可能.

I'm trying to accomplish the above logic with the updateOne method and a aggreagtion pipeline however I'm not sure if this is even possible.

我目前拥有的是返回"Unrecognized pipeline stage name: '$cond'"

const updateUpvotes = {
  $cond: {
    if: { $elemMatch: { upvotes: ObjectID(userId) } },
    then: { $pull: { upvotes: ObjectID(userId) } },
    else: { $addToSet: { upvotes: ObjectID(userId) } }
  }
};

db.collection(collectionName).updateOne({
   commentId: ObjectID('5e5983102328a83d1a4b541f')
}, [updateUpvotes])

我是否对整个功能有过多的考虑?我想1.问题可以通过简单地创建CommentReputation文档(同时创建Comment文档的同时包含空白upvotes和downvotes)来解决.

Am I overthinking the whole feature? I guess the 1. problem can be solved by simply creating a CommentReputation document (with empty upvotes and downvotes at the same time the Comment document is being created.

是否有更好的方法?我很想让它在单个查询请求中运行.也许你们中的某人实现了类似的功能，并且可以给我一些提示.

Is there a better way of doing this? I would love to have it working inside a single query request. Maybe someone of You guys implemented a similar feature and can give me some hints on this one.

推荐答案

您可以通过以下管道更新来完成此操作，但它要求存在upvotes和downvotes数组.即使只是空的.

you can do it with the following pipeline update but it requires that the upvotes and downvotes arrays exist. even if it's just empty.

var comment_id = ObjectId("5e5983102328a83d1a4b541f");
var user_id = ObjectId("5e5983102328a83d1a4b53e5");

db.commentReputation.update(
    {
        commentId: comment_id
    },
    [
        {
            $set: {
                upvotes: {
                    $cond: [
                        { $in: [user_id, '$upvotes'] },
                        { $setDifference: ['$upvotes', [user_id]] },
                        { $setUnion: ['$upvotes', [user_id]] }
                    ]
                }
            }
        },
        {
            $set: {
                downvotes: {
                    $cond: [
                        { $in: [user_id, '$downvotes'] },
                        { $setDifference: ['$downvotes', [user_id]] },
                        '$downvotes'
                    ]
                }
            }
        }
    ]
);

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