MongoDB aggregate for Arithmetic Operations - Subdocument Field

Question

We have a aggregate query projecting few sub documents. We wanted to apply a few arithmetic operations like Sum and Product on these projected values.

Aggregation Query --

Item.aggregate([
        { $unwind: '$dummy'},       
        { $match: {'dummy.storage': {$gt: 0}} },
        { $group: {_id: '$_id',
                    dummy: {$push: '$dummy'},
                    original_y: { $first: "$original_y" },
                    new_y: { $first: "$new_y" },

        }},
        {$project:{
                  original_y: 1, new_y: 1,
                  tallyAmount: {$sum: ["$new_y","$original_y"] }
                }
        },
     ]
    )
    .exec(function(err, results){   
        if(err)
        {
            console.log("Error : " + err);
            return res.json ({error: "Error"});

        }
        else if(!(results) || results == null || results.length == 0)
        {
            console.log("No Results Found");
            return res.json ({error: "No Results Today"});

        }else{

            res.send(results);
        }
    });

This gives an error stating invalid operator '$sum'

What should we do to get the sum of original_y and new_y in the $project?

EDIT

Documents:

{
 id:1,
 original_y: 200,
 new_y: 140,
 dummy: [
  {id:1, storage:2, cost: 10},
  {id:2, storage:0, cost: 20},
  {id:3, storage:5, cost: 30},
  ]
}

OUTPUT EXPECTED:

 {
     id:1,
     original_y: 200,
     new_y: 140,
     dummy: [
      {id:1, storage:2, cost: 10, tallyAmount: 34},
      {id:3, storage:5, cost: 30, tallyAmount: 11.33},
      ]
    }

where, tallyAmount = (original_y + new_y) / cost

ERROR: can't add an expression for a subfield of dummy because there is already an expression that applies to the whole field

Answer 1

Without much details of your document schema and the expected aggregation result, I would suggest you try the following aggregation as I believe you need the $add operator rather than the $sum operator. Note that the $sum operator is only applicable to the $group operator. With the $add operator, it adds up two numbers/fields together and the result is stored in a new field with the $project operator:

Item.aggregate([
    { "$match": { "dummy.storage": { "$gt": 0 } } },
    { "$unwind": "$dummy" },   
    { "$group": { 
        "_id": "$_id", 
        "original_y": { "$first": "$original_y" },
        "new_y": { "$first": "$new_y" }
    } },
    { "$project": {
        "original_y": 1, 
        "new_y": 1,
        "tallyAmount": { "$add": [ "$new_y", "$original_y" ] }
    } }
]).exec(callback);

-- UPDATE --

To satisfy the condition tallyAmount = (original_y + new_y) / cost, you should use the $add and $divide arithmetic operators in your $project operator pipeline stage thus your final aggregation pipeline would look like this:

Item.aggregate([
    { "$match": { "dummy.storage": { "$gt": 0 } } },
    { "$unwind": "$dummy" },
    {
        "$project": {
            "original_y": 1, 
            "new_y": 1,
            "dummy.id": "$dummy.id",
            "dummy.storage": "$dummy.storage",
            "dummy.cost": "$dummy.cost",
            "dummy.tallyAmount": {
                "$divide": [
                    { "$add": ["$new_y","$original_y"] },
                    "$dummy.cost"
                ]
            }
        }
    },
    {
        "$group": {
            "_id": "_$id",
            "original_y": { "$first": "$original_y" },
            "new_y": { "$first": "$new_y" },
            "dummy": {
                "$addToSet": "$dummy"
            }
        }        
    }
 ]).exec(callback);