{
  "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": ""
    }
  ]

}

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

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

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

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

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