以下云函数查询语句，在云开发后台测试没有问题，但在小程序端总是报错，是什么原因？db.collection('stitem').aggregate()

.lookup({

from: 'article',

localField: 'article_id',

foreignField: '_id',

as: 'article',

}).replaceRoot({

newRoot: $.mergeObjects([ $.arrayElemAt(['$article', 0]), '$$ROOT' ])

}).project({

article: 0

})

.end()

小程序端报错如下：

Error: cloud.callFunction:fail Error: errCode: -504002 functions execute fail | errMsg: ReferenceError: $ is not defined

    at get_st_detail (/var/user/index.js:71:14)

    at Runtime.exports.main [as handler] (/var/user/index.js:15:14)

    at Runtime.handleOnce (/var/runtime/node16/Runtime.engine.js:135:21)

    at Timeout._onTimeout (/var/runtime/node16/Runtime.engine.js:54:12)

    at listOnTimeout (node:internal/timers:557:17)

    at processTimers (node:internal/timers:500:7) (callId: 1714035431672-0.21178932569511377) (trace: 16:57:11 start->16:57:12 system error (Error: errCode: -504002 functions execute fail | errMsg: ReferenceError: $ is not defined

    at get_st_detail (/var/user/index.js:71:14)

    at Runtime.exports.main [as handler] (/var/user/index.js:15:14)

    at Runtime.handleOnce (/var/runtime/node16/Runtime.engine.js:135:21)

    at Timeout._onTimeout (/var/runtime/node16/Runtime.engine.js:54:12)

    at listOnTimeout (node:internal/timers:557:17)

    at processTimers (node:internal/timers:500:7)), abort)

    at R (<anonymous>:1:245532)

    at <anonymous>:1:279958(env: macOS,mp,1.06.2402040; lib: 3.4.3)