How to get object value with dynamic key in $project in mongoDB

Question

I want to retrieve a value from an object with a dynamic key

[
  {
    "_id": 1,
    "item": "sweatshirt",
    "price": {
      "INR": 45.99
    },
     "currency": 'INR'
  }
]


db.collection.aggregate([
  {
    "$project": {
      "pricenew": "$price.currency"
    }
  }
])

If I do price.INR it will work fine but here I have currency dynamic, so I want something like price.currency but here currency is coming like "INR" and it gives no data.

I really appreciate any help you can provide.

Answer 1

You need to convert the price object to an array using $objectToArray , filter it and then convert it back, like so:

db.collection.aggregate([
  {
    $replaceRoot: {
      newRoot: {
        "$mergeObjects": [
          {
            "$arrayToObject": {
              $map: {
                input: {
                  $filter: {
                    input: {
                      "$objectToArray": "$price"
                    },
                    cond: {
                      $eq: [
                        "$$this.k",
                        "$currency"
                      ]
                    }
                  }
                },
                in: {
                  k: "pricenew",
                  v: "$$this.v"
                }
              }
            }
          },
          {
            _id: "$_id"
          }
        ]
      }
    }
  }
])

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