Question

I have several numeric fields that I need to aggregate. Let’s say my document is structured as follows:

_id: 1234,
numValue1: 10,
numValue2: 20,
numValue3: 30,
numValue4: 40

If I wanted to add a computed field to my pipeline using one of the numeric fields, I could do something like this:

db.myCollection.aggregate(
{ 
  $project : {
    someComputedField : { $add:["$numValue1", 15] }
  }
})

If I wanted to add a computed field based on two of these fields, I know I could do something like this:

db.myCollection.aggregate(
{ 
  $project : {
    someComputedField : { $add:["$numValue1", "$numValue2"] }
  }
})

Now, my question is what if I needed to do $numValue1 + $numValue2 + $numValue3 + $numValue4?

Or even more interestingly $numValue1 * ($numValue2 + $numValue3) / $numValue4?

Answer 1

$add can accept multiple values, so the first case would be:

someComputedField: {$add: ['$numValue1', '$numValue2', '$numValue3', '$numValue4']}

And you can nest operators, so the second case would be:

someComputedField: {
    $divide: [
        {$multiply: [
            '$numValue1', {$add: ['$numValue2', '$numValue3']}
        ]},
        '$numValue4'
    ]
}