MongoError:无法识别的表达式'$last‘(“版本”："5.0.4")

Question

当我试着在总体上投射

我正在使用这个MongoDB版本(“版本”："5.0.4")

                {
                    $project: {
                        _id: 1,
                        feeling: 1,
                        updated_date: 1,
                        created_date: 1,
                        activity_type: 1,
                        activityCount: { $size: "$activity" },
                        activity_count: 1,
                        activity_tipped_count: 1,
                        activity_upvote_count: 1,
                        activity: { $last: "$activity" },
                        product: 1,
                        property: 1,
                        shared_post: 1,
                        post: 1,
                        user: 1,
                        rss_delete: 1,
                    },
                },

它给出了这个错误

MongoError: Unrecognized expression '$last'
    at Connection.<anonymous> (/Users/mac/Documents/WORK DISK/A PROJECTS/OutSourcing/Revstance-node-v3 /revstancev3/node_modules/mongodb/lib/core/connection/pool.js:453:61)
    at Connection.emit (node:events:390:28)
    at Connection.emit (node:domain:475:12)
    at processMessage (/Users/mac/Documents/WORK DISK/A PROJECTS/OutSourcing/Revstance-node-v3 /revstancev3/node_modules/mongodb/lib/core/connection/connection.js:456:10)
    at Socket.<anonymous> (/Users/mac/Documents/WORK DISK/A PROJECTS/OutSourcing/Revstance-node-v3 /revstancev3/node_modules/mongodb/lib/core/connection/connection.js:625:15)
    at Socket.emit (node:events:390:28)
    at Socket.emit (node:domain:475:12)
    at addChunk (node:internal/streams/readable:315:12)
    at readableAddChunk (node:internal/streams/readable:289:9)
    at Socket.Readable.push (node:internal/streams/readable:228:10)
    at TCP.onStreamRead (node:internal/stream_base_commons:199:23) {
  ok: 0,
  code: 168,
  codeName: 'InvalidPipelineOperator'
}

谢谢，任何帮助对我来说都很重要。

Answer 1

我猜这是驱动程序未能区分$last聚合累加器和$arrayElemAt(-1)的$last别名。根据https://docs.mongodb.com/manual/reference/operator/aggregation/last/的文档，累积运算符在$project阶段无效。

也许试着用这个来代替它：

activity: { $arrayElemAt: [ "$activity", -1 ] }