MongoDB array[] sum-function

Question

I'm trying to count sum of all balances for every currency in database. I had to group it by currency but $sum was returning an exception. Any ideas how to fix it ?

exception: query failed: (Location40237) The $sum accumulator is a unary operator

    db.getCollection("people").aggregate([
    {
        $group: {
            _id: { currency:  "$credit.currency" },
            sumCur: { $sum: [ { $toDouble:  "$credit.balance" }]}

        }
    }
])

Sample of data model:

{ 
    "_id" : ObjectId("5ea970747cd4ac05869977ec"), 
    "sex" : "Male", 
    "first_name" : "Wayne", 
    "last_name" : "Fields", 
    "job" : "Speech Pathologist", 
    "email" : "[email protected]", 
    "location" : {
        "city" : "Oyo", 
        "address" : {
            "streetname" : "Beilfuss", 
            "streetnumber" : "860"
        }
    }, 
    "description" : "vulputate justo in blandit ultrices enim lorem ipsum dolor sit amet consectetuer adipiscing elit proin interdum mauris", 
    "height" : "152.38", 
    "weight" : "66.81", 
    "birth_date" : "1990-02-21T02:55:03Z", 
    "nationality" : "Nigeria", 
    "credit" : [
        {
            "type" : "switch", 
            "number" : "6759888939100098699", 
            "currency" : "COP", 
            "balance" : "5117.06"
        }
    ]
}

mickl · Accepted Answer · 2020-05-01 23:51:43Z

2

$sum expects a scalar, numeric value while you're passing an array. You need to use $unwind first since different array elements may also have different currencies:

db.collection.aggregate([
    { $unwind: "$credit" },
    {
        $group: {
            _id: {
                currency: "$credit.currency"
            },
            sumCur: {
                $sum: { $toDouble: "$credit.balance" }
            }
        }
    }
])

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answered May 1, 2020 at 23:51

mickl

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