MongoDB Aggregate with mergeObjects

I have a statistics collection that contains the statistics for every day, so every day I have a different document, currently the only valid statistic that I have is "accuracy".

Accuracy is a Record<string, number> an object { [string]: number } basically, the keys are dynamic so I don't know beforehand what keys are going to be, but for this instance, there are valid keys: low, medium, high.

db.getCollection('statistics').aggregate( [
    {
        $group: {
            _id: null,
            accuracy: { $push: "$accuracy" }
        }
    },
] )

Which results in:

{
    "_id" : null,
    "accuracy" : [ 
        {
            "low" : 4
        }, 
        {
            "low" : 6
        }
    ]
}

But I don't want them to be both in different indexes, I want them to be accumulated and added together as follows:

{
    "_id" : null,
    "accuracy" : {
        "low" : 10
    }
}

So that led me to find this:

db.getCollection('statistics').aggregate( [
    {
        $group: {
            _id: null,
            accuracy: { $mergeObjects: "$accuracy" }
        }
    },
] )

The $mergeObjects, but with that another issue arises, it just merges and overrides the values and the output is wrong:

{
    "_id" : null,
    "accuracy" : {
        "low" : 6
    }
}

If anyone has any idea how to increment that count that would be much appreciated, $mergeObjects plus some sort of accumulation $add would be ideal but I couldn't find any useful resource in the docs for my problem.

Solution

$set - Convert the key-value pair from the accuracy field to an array of objects consisting of k and v fields.
$unwind - Descontruct the accuracy array into multiple documents.
$group - Group by accuracy.k and perform sum with accuracy.v.
$group - Group all documents, construct an array of objects consisting of k and v fields, convert the array into a key-value pair, and add into the accuracy array.

db.collection.aggregate([
  {
    $set: {
      accuracy: {
        $objectToArray: "$accuracy"
      }
    }
  },
  {
    $unwind: "$accuracy"
  },
  {
    $group: {
      _id: "$accuracy.k",
      value: {
        $sum: "$accuracy.v"
      }
    }
  },
  {
    $group: {
      _id: null,
      accuracy: {
        $push: {
          $arrayToObject: [
            [
              {
                k: "$_id",
                v: "$value"
              }
            ]
          ]
        }
      }
    }
  }
])

Demo @ Mongo Playground