MongoDB mathematic operations with array elements

Question

We have collection with documents like:

{ 
    "_id" : <ObjectId>, 
    "player" : <String>, 
    "params" : [
        ...,
        {
            "name" : "kills", 
            "value" : 10
        },
        {
            "name" : "deaths", 
            "value" : 10
        },
        {
            "name" : "assists", 
            "value" : 10
        },
        ...
    ]
}

Is there any way to get list of all players with calculated KDA? KDA=(kills+assists)/deaths

For example, like this:

[{
    "_id" : <ObjectId>,
    "player" : <String>,
    "kda" : 2.5
},
...
]

And how to execute queries on resulted collection? To find all players with KDA>10 etc. Thanks!

Would be much better if you had looked up this question, tried out something and share the code you have. — vahdet
– vahdet, Commented Jul 11, 2018 at 8:23

dnickless · Accepted Answer · 2018-07-11 10:05:12Z

If the order in your params array is expected to be identical across all elements one way would be like this:

db.collection.aggregate({
    $addFields: {
        "kda": {
            $divide: [
                { $sum: [
                    { $arrayElemAt: [ "$params.value", 0 ] },
                    { $arrayElemAt: [ "$params.value", 2 ] }
                ]},
                { $arrayElemAt: [ "$params.value", 1 ] } ] 
        }
    }
})

If the order is not known or fixed you can do this:

db.collection.aggregate({
    $addFields: {
        "kda": {
            $divide: [
                { $sum: [
                    { $arrayElemAt: [ "$params.value", { $indexOfArray: [ "$params.name", "kills" ] } ] },
                    { $arrayElemAt: [ "$params.value", { $indexOfArray: [ "$params.name", "assists" ] } ] }
                ]},
                { $arrayElemAt: [ "$params.value", { $indexOfArray: [ "$params.name", "deaths" ] } ] } ] 
        }
    }

})

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