Find only one field value in an array by match expression

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1

Hello for all!

I’m new in mongosh, and I’m stuck in a task with finding value of field in array (only one element by match). I have data like:


[
  {
    "key": 1,
    "obj": [
      {
        "k": "hohoh",
        "v": "dododo"
      },
      {
        "k": "nonon",
        "v": "tratata"
      },
      {
        "k": "Descr",
        "v": "This is value What I Only NEED!"
      },
      {
        "k": "bbzbzbzbzb",
        "v": "kkkO"
      },
      
    ]
  },
  {
    "key": 2,
    "obj": [
      {
        "k": "hohoh",
        "v": "dododo"
      },
      {
        "k": "nonon",
        "v": "tratata"
      },
      {
        "k": "Descr",
        "v": "There is cool, but key = 2 and this value must be ignored"
      },
      {
        "k": "bbzbzbzbzb",
        "v": "kkkO"
      },
      
    ]
  }
]

I need query like SELECT obj.v WHERE key IS 1 AND obj.k IS “Descr”.
I try use query like that : 




db.collection.find({
  "key": 1,
  "obj": {
    "$elemMatch": {
      "k": "Descr"
    }
  }
},
{
  "obj.v": 1
})
but there return fields obj.v from every element of array and I need value of obj.v in element where obj.k = Descr
2

I do not know if you can get just the v field but you can get the matching array element with the following projection.

{ "obj.$" : 1 }