Mongodb aggregation: insert nested fields in $group stage

Question

I'm trying to create some nested fields in $group stage of aggregation. I get the error that "stats" must be an accumulator. Is there any way to insert these nested fields?

model.aggregate( [ {
  $group: {
    _id: '$groupName,
    nestedFields: {
      field1: {
        $sum: { $multiply: [ '$num1', '$num2'] }
      },
      field2: { $sum: '$num3' }
    }
  }
} ] )

Any help is greatly appreciated.

edit: some sample data INPUT:

{
  "groupName": 1
  "num1": 2,
  "num2": 4,
  "num3": 8,
}
{
  "groupName": 1
  "num1": 2,
  "num2": 7,
  "num3": 1,
}
{
  "groupName": 2
  "num1": 1,
  "num2": 3,
  "num3": 5,
}
{
  "groupName": 2
  "num1": 6,
  "num2": 1,
  "num3": 2,
}

should OUTPUT through the aggregation:

{
  "_id": 1,
  "nestedFields": {
    "field1": 22,      // 2*4 + 2*7
    "field2": 9,       // 8 + 1
  }
},
{
  "_id": 2,
  "nestedFields": {
    "field1": 9,      
    "field2": 7,       
  }
}

You need to do it in the next stage with $addFields or $project — Valijon
– Valijon, Commented Mar 30, 2020 at 7:22
can you share your request data and response data ? if not solve your problem — Mahesh Bhatnagar
– Mahesh Bhatnagar, Commented Mar 30, 2020 at 7:42
Basicaly i have to save the values in common fields in $group, nest them in $addFields and remove the original ones in $project. Isnt there a better way @Valijon ? — KFaidon K.
– KFaidon K., Commented Mar 30, 2020 at 7:42
@MaheshBhatnagar It's too much. I will edit the question to make it a bit simpler. — KFaidon K.
– KFaidon K., Commented Mar 30, 2020 at 7:45

Valijon · Accepted Answer · 2020-03-30 11:47:29Z

1

During $group stage, MongoDB does not allow create nested fields.

Workaround: You need to store aggregated values in ordinary fields and in the next stage, transform into desired result with $project operator.

model.aggregate([
  {
    $group: {
      _id: "$groupName",
      field1: {
        $sum: {
          $multiply: [
            "$num1",
            "$num2"
          ]
        }
      },
      field2: {
        $sum: "$num3"
      }
    }
  },
  {
    $project: {
      nestedFields: {
        field1: "$field1",
        field2: "$field2"
      }
    }
  }
])

MongoPlayground

answered Mar 30, 2020 at 11:47

Valijon

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