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)