Find Specific Field in nested Object with mongoose

Question

I Have a document like this :

{
  "_id": {
    "$oid": "64494e6a2f90fa940d5a406c"
  },
  "theme": "Harry Potter",
  "date": {
    "$date": "2023-04-26T22:00:00.000Z"
  },
  "quiz": [
    {
      "question": "Qui a une cicatrice sur le front ?",
      "responses": [
        {
          "nbVote": 1,
          "label": "Ron",
          "isCorrect": false,
          "_id": {
            "$oid": "64494e6a2f90fa940d5a405a"
          }
        },
        {
          "nbVote": 0,
          "label": "Hermione",
          "isCorrect": false,
          "_id": {
            "$oid": "64494e6a2f90fa940d5a405b"
          }
        },
        {
          "nbVote": 0,
          "label": "Malfoy",
          "isCorrect": false,
          "_id": {
            "$oid": "64494e6a2f90fa940d5a405c"
          }
        },
        {
          "nbVote": 0,
          "label": "Harry",
          "isCorrect": true,
          "_id": {
            "$oid": "64494e6a2f90fa940d5a405d"
          }
        }
      ],
      "_id": {
        "$oid": "64494e6a2f90fa940d5a405e"
      }
    },
    {
      "question": "De quel maison fait partie Harry Potter ?",
      "responses": [
        {
          "nbVote": 0,
          "label": "Poufsouffle",
          "isCorrect": false,
          "_id": {
            "$oid": "64494e6a2f90fa940d5a4063"
          }
        },
        {
          "nbVote": 0,
          "label": "Serpentard",
          "isCorrect": false,
          "_id": {
            "$oid": "64494e6a2f90fa940d5a4064"
          }
        },
        {
          "nbVote": 0,
          "label": "Serdaigle",
          "isCorrect": false,
          "_id": {
            "$oid": "64494e6a2f90fa940d5a4065"
          }
        },
        {
          "nbVote": 1,
          "label": "Gryffondor",
          "isCorrect": true,
          "_id": {
            "$oid": "64494e6a2f90fa940d5a4066"
          }
        }
      ],
      "_id": {
        "$oid": "64494e6a2f90fa940d5a4067"
      }
    }
  ],
  "nbPlayers": 0,
  "__v": 0
}

And I would like get the only the nbVote field in my request, I try like that :

async getNbVote(id: string) {
    return this.dailyQuizModel
      .findOne(
        { 'quiz.responses._id': id },
        {
          'quiz.responses.nbVote': 1,
        },
      )
      .lean()
      .exec();
  }

For example if id === 64494e6a2f90fa940d5a405a, mongoose will return this :

{
  _id: {
  },
  quiz: [
    {
      responses: [
        {
          nbVote: 1,
        },
        {
          nbVote: 0,
        },
        {
          nbVote: 0,
        },
        {
          nbVote: 0,
        },
      ],
    },
    {
      responses: [
        {
          nbVote: 0,
        },
        {
          nbVote: 0,
        },
        {
          nbVote: 0,
        },
        {
          nbVote: 1,
        },
      ],
    },
  ],
}

But I only want the nbVote from the Id I gave, I don't know what can i do... Thanks for help ! I Try with arrayFilters, and Aggregate but with no success

Answer 1

Here's one way you could do it. Comments are in the aggregation pipeline.

db.collection.aggregate([
  {// find doc with correct quiz.responses._id
    "$match": {
      "quiz.responses._id": ObjectId("64494e6a2f90fa940d5a405a")
    }
  },
  {
    "$project": {
      // only want nbVote
      "_id": 0,
      "nbVote": {
        "$reduce": {
          // input is an array of arrays
          "input": "$quiz.responses",
          "initialValue": null,
          "in": {
            "$reduce": {
              // this input is an aray of objects
              "input": "$$this",
              // initialValue is set as outer reduce $$value
              "initialValue": "$$value",
              "in": {
                "$cond": [
                  // is this the correct response?
                  {"$eq": ["$$this._id", ObjectId("64494e6a2f90fa940d5a405a")]},
                  // Yes! Get nbVote!
                  "$$this.nbVote",
                  // No.  Keep current $$value
                  "$$value"
                ]
              }
            }
          }
        }
      }
    }
  }
])

Example output:

[
  {
    "nbVote": 1
  }
]

Try it on mongoplayground.net.

Rather than using aggregate, you could use find with just minor modifications. See this mongoplayground.net example that uses find instead.