Question 115409 in Mongodb Question posted in
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MongoDB – How to use $toInt in $sum of a field in an array?

Can you guys help me with this problem?

Example:
Data:

{
      _id: "63f69650e820990f82dca60b",
      ...,
      transactions: [
        {
          ...,
          fee: "100",
          ...
        },
        {
          ...,
          fee: "200",
          ...
        }
      ]
    }

I want to add a total_fee field to calculate total fee of all the transactions in a document.

I tried using

$addfields: {
  total_fee: {
    $sum: {$toInt: "$transactions.fee"}
  }
}

But mongo return error about can’t convert array to int

Tags:

2

Answers


  1. 0

    When you specify "$transactions.fee", it resolves to an array of strings. $toInt doesn’t accept an array.
    You use $reduce to iterate through the array and convert each string to integer, before adding.

    db.collection.aggregate([
  {
    $addFields: {
      total_fee: {
        $reduce: {
          input: "$transactions.fee",   //1. Input is the array of strings
          initialValue: 0,              //2. Start 
          in: {                         //3. iterate though the array
            $add: [
              "$$value",                //5. add the converted string to intermediate result, accessed using $value
              {$toInt: "$$this"}        //4. use $this to access current value of the iteration, convert it to int
            ]
          }
        }
      }
    }
  }
])

    Demo

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  2. 0

    Using $reduce is an overkill in my opinion. $map should be simpler and is certainly faster:

    db.collection.aggregate([
  {
    $addFields: {
      total_fee: {
        $sum: {
          $map: {
            input: "$transactions.fee",
            in: { $toInt: "$$this" }
          }
        }
      }
    }
  }
])

    Of course, the proper solution is to store numeric values as number rather than strings.

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