How to get length of array in mongodb $group operator?

Question

my collection Data is like this:

[
  { "_id": 1, "s": "a", "kc": 100 },
  { "_id": 2, "s": "a", "kc": 101 },
  { "_id": 3, "s": "a", "kc": 102 },
  { "_id": 4, "s": "a", "kc": 103 },
  { "_id": 5, "s": "b", "kc": 200 },
  { "_id": 6, "s": "b", "kc": 201 },
  { "_id": 7, "s": "b", "kc": 202 },
  { "_id": 8, "s": "b", "kc": 203 }
]

I use this aggregation:

db.collection.aggregate([
  {
    "$group": {
      _id: "$s",
      kcList: {
        "$push": "$kc"
      }
    }
  }
])

and the result was as below:

[
  {
    "_id": "a",
    "kcList": [
      100,
      101,
      102,
      103
    ]
  },
  {
    "_id": "b",
    "kcList": [
      200,
      201,
      202,
      203
    ]
  }
]

I want to change my Query In a way that when grouping data in kcList, each item multiplied to its array index.

I want the result as below:

[
  {
    "_id": "a",
    "kcList": [
      100 * 0,
      101 * 1,
      102 * 2,
      103 * 3
    ]
  },
  {
    "_id": "b",
    "kcList": [
      200 * 0,
      201 * 1,
      202 * 2,
      203 * 3
    ]
  }
]

so I wrote this query:

db.collection.aggregate([
  {
    "$group": {
      _id: "$s",
      kcList: {
        "$push": {
          "$multiply": [
            "$kc",
            {
              $cond: {
                if: {
                  $isArray: "$kcList"
                },
                then: {
                  $size: "$kcList"
                },
                else: 0
              }
            }
          ]
        }
      }
    }
  }
])

But the result is wrong:( do you have any idea to solve this?

Mongo playground

The expected output which you provided is not clear. what do you mean by 100 * 0, 101 * 1? — hhharsha36
– hhharsha36, Commented Jun 8, 2021 at 9:05
100 , 101 , 102 , 103 is the 'kc' value and 0 , 1 , 2 , 3 is the array item index — mehdi parastar
– mehdi parastar, Commented Jun 8, 2021 at 9:10

turivishal · Accepted Answer · 2021-06-08 09:16:55Z

1

Add new stage after group stage,

$range to create an array of numbers from 0 to length of kcList array using $size
$map to iterate loop of the above range
$arrayElemAt to get element of range position in kcList array
$multiply to multiply range number with kcList value returned from the above operation

db.collection.aggregate([
  {
    $group: {
      _id: "$s",
      kcList: { $push: "$kc" }
    }
  },
  {
    $project: {
      kcList: {
        $map: {
          input: { $range: [0, { $size: "$kcList" }] },
          in: {
            $multiply: [
              { $arrayElemAt: ["$kcList", "$$this"] },
              "$$this"
            ]
          }
        }
      }
    }
  }
])

Playground

answered Jun 8, 2021 at 9:16

turivishal

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