Question 167678 in Javascript Question posted in
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Javascript – How to subract the nested array object values in mongo

I would like to know how to subtract the nested array object field values in mongodb.

{
  Active: [
    {min: 1, energy: 24},
    {min: 2, energy: 34},
    {min: 3, energy: 65}
  ]
}

Expected output:
Difference: 65-24 = 41

I have tried this:

{
  $project: {
    Difference: {
      $subtract: ["$active.3.energy","$active.0.energy"]
    }
  }
}

query2:

{
  $project: {
    Difference: {
      $subtract: ["$active.$[n].energy","$active.0.energy"]
    }
  }
}
Tags:

2

Answers


  1. 0

    Here’s one way you could do it.

    db.collection.aggregate([
  {
    "$project": {
      "Difference": {
        "$subtract": [
          {"$arrayElemAt": ["$Active.energy", 2]},
          {"$arrayElemAt": ["$Active.energy", 0]}
        ]
      }
    }
  }
])

    Try it on mongoplayground.net.

    Login or Signup to reply.
  2. 0

    Assume that the Active array can be of arbitrary length and we wish to get the difference between the greatest and least value of energy. Here’s a solution:

    db.foo.aggregate([
    // Ensure sorted ordering of array:                                          
    {$addFields: {
        'Active': {$sortArray: {'input':'$Active', 'sortBy': { energy: 1 }}}
    }}              

    // $arrayElemAt is 0-based, so need $size:'$Active' - 1:                     
    ,{$project: {
        'difference': {$subtract: [
            {$arrayElemAt:['$Active.energy', {$subtract:[{$size:'$Active'},1]} ]},
            {$arrayElemAt:['$Active.energy',0]} ] }
    }}
]);
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