代码如下：想查询num大于1的数据 

return db.collection('amount').aggregate()

    .group({

      _id: '$openid',

      num: $.sum(1)

    })

    .match({num:_.gt(1)})

    .end()

    .then((result)=>{

      console.log(result)

    })

报错如下：

{"errorCode":1,"errorMessage":"user code exception caught","stackTrace":"Error: collection.aggregate:fail -502001 database request fail. [FailedOperation] Failed operation. Please check your request, but if the problem persists, contact us.\n at Object.toSDKError (/var/user/node_modules/wx-server-sdk/index.js:8094:19)\n at Object.checkError (/var/user/node_modules/wx-server-sdk/index.js:1422:23)\n at Promise (/var/user/node_modules/wx-server-sdk/index.js:1161:33)\n at process._tickCallback (internal/process/next_tick.js:68:7)","statusCode":430}