MongoDB - How to sum 2 string values in array after their conversion to integer?

Question

I've been breaking my head on this for a while now:

[
  {
    "_id": "12sdsd",
    "TotalStudent": [
      "10"
    ]
  },
  {
    "_id": "22fdf",
    "TotalStudent": [
      "20"
    ]
  }
]

I need to sum the two values after conversion from String to Integer. This doesn't work:

db.collection.aggregate([
  {
    $group: {
      _id: "",
      TotalStudents: {
        $sum: {
          $toInt: "$TotalStudent.0"
        }
      }
    }
  }
])

See the playground, what am I doing wrong? https://mongoplayground.net/p/Evm3tJUGmKa

prasad_ · Accepted Answer · 2020-11-18 03:07:57Z

1

"$TotalStudent.0" - This is not a valid syntax with aggregation queries. When working with array fields in aggregations, use Aggregation Array Operators. The operator to get an element of an array by its index is $arrayElemAt. So, the following $group stage will work fine:

{ $group: { 
    _id: "", 
    TotalStudents: { $sum: { $toInt: { $arrayElemAt: [ "$TotalStudent", 0 ] } } } 
} }

answered Nov 18, 2020 at 3:07

prasad_

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