How to convert deeply nested array with one object to object in mongodb aggregation?

The requirement is to convert all sales array properties to objects with their first elements’ quantity value. The collection name is branches and the example data is below.

[
{
    "_id": 111,
    "recs": [
        {
            "date": "01-01-2021",
            "sales": [
                {
                    "quantity": {
                        "P": 14,
                        "K": 0,
                        "U": 0
                    }
                }
            ]
        },
        {
            "date": "02-01-2021",
            "sales": [
                {
                    "quantity": {
                        "P": 23,
                        "K": 0,
                        "U": 0
                    }
                }
            ]
        }
    ]
},
{
    "_id": 21,
    "recs": [
        {
            "date": "01-01-2021",
            "sales": [
                {
                    "quantity": {
                        "P": 11,
                        "K": 0,
                        "U": 0
                    }
                }
            ]
        },
        {
            "date": "02-01-2021",
            "sales": [
                {
                    "quantity": {
                        "P": 31,
                        "K": 0,
                        "U": 0
                    }
                }
            ]
        }
    ]
}]

The expected response is below.

[
{
    "_id": 111,
    "recs": [
        {
            "date": "01-01-2021",
            "sale": {
                "P": 14,
                "K": 0,
                "U": 0
            }
        }
    ]
}]

I tried $map, $addFields but no way I couldn’t find a solution.

>Solution :

In $map, you need:

1. $first to query the first sales.quantity object in recs.sales array.
2. $mergeObject to merge the current document in recs array with the document from (1).

db.collection.aggregate([
  {
    "$project": {
      _id: 1,
      recs: {
        "$map": {
          "input": "$recs",
          "in": {
            "$mergeObjects": [
              "$$this",
              {
                sales: {
                  $first: "$$this.sales.quantity"
                }
              }
            ]
          }
        }
      }
    }
  }
])

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