Mongodb-未知的顶级运算符:$ elemMatch" [英] Mongodb - unknown top level operator: $elemMatch"

问题描述

我有一个收藏，其中包含一些文档，

I have a collection which contains some documents as follows,

{
  "transactionId": 3432423,
  "reviews": [
    {
      "fromUser": {
        "userId": "5236aa1acd6e"
      },
      "toUser": {
        "userId": "0ec8db9544cc"
      },
      "rating": 4.3,
      "comment": ""
    },
    {
    "toUser": {
        "userId": "5236aa1acd6e",
        "email": "[e-mail]",
        "firstName": "your",
        "lastName": "life"
      },
      "fromUser": {
        "userId": "0ec8db9544cc",
        "email": "[e-mail]",
        "firstName": "my",
        "lastName": "life"
      },
      "rating": 4.3,
      "comment": ""
    }
  ]

}

我需要检查文档中是否存在子文档审阅.我已经使用了这个查询，

i need to check if subdocument reviews present inside the document. i have used this query,

db.getCollection('_testCollection').find({  "$elemMatch": { "reviews": { "$exists": false } }})

它抛出一个错误，说，

"errmsg" : "unknown top level operator: $elemMatch",

推荐答案

$elemMatch或$exists都不是您要用来确定reviews是否为空数组的东西.

Neither $elemMatch or $exists is what you want to use to determine if reviews is an empty array.

相反，可以将其与[]进行比较:

Instead, either compare against []:

db.test.find({reviews: []})

或使用$size运算符:

db.test.find({reviews: {$size: 0}})

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