Mongodb - unknown top level operator: $elemMatch"

Question

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": "yourlife@gmail.com",
        "firstName": "your",
        "lastName": "life"
      },
      "fromUser": {
        "userId": "0ec8db9544cc",
        "email": "mylife@gmail.com",
        "firstName": "my",
        "lastName": "life"
      },
      "rating": 4.3,
      "comment": ""
    }
  ]

}

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

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

it throws an error saying,

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

`db.getCollection('_testCollection').find({ "reviews" : { "$elemMatch": { "$exists": false } }})` It must be like this way because reviews is key. for finding in array @JohnnyHK is right. You dont need to use $elemMatch. — Yogesh Patel, Apr 28 '17 at 04:40
@JohnnyHK it does not return any results. i have a doc wiht reviews:[] . it does not retun that — Sajeetharan, Apr 28 '17 at 07:00

score 4 · Accepted Answer · answered Apr 28 '17 at 13:50

4

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

Instead, either compare against []:

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

Or use the $size operator:

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

answered Apr 28 '17 at 13:50

JohnnyHK

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score 0 · Answer 2 · answered Apr 28 '17 at 05:18

From the Docs :-

The $elemMatch operator matches documents that contain an array field with at least one element that matches all the specified query criteria.

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

$exists :- true or false // depend on requirement

Thanks

Mongodb - unknown top level operator: $elemMatch"

2 Answers2

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