MongoDB slice - remove last n elements from nested arrays

Question

Supposing I have the following array:

{
   data: [1, 0, 4, 0, 0, 4, 1, 3, 0, 1, 0, 2, 2, 0, 1, 1, 0, 2, 0, 4, 1, 1, 0, 1, 1, 0]
}

if I want to select all the elements, except the last 3, I can use the solution proposed here

db.collection.aggregate([
  {
    $project: {
      "_id": 0,
      "data": {
        "$slice": [ 
          "$data",
          0,
          {
            $subtract: [ { $size: "$data" }, 3 ]
          }
        ]
      }
    }
  }
])

I can get the desired output:

[1, 0, 4, 0, 0, 4, 1, 3, 0, 1, 0, 2, 2, 0, 1, 1, 0, 2, 0, 4, 1, 1, 0]

How can I do the same thing for concatenated/nested arrays like:

 {
       data: [[1, 0, 4, 0, 0, 4, 1], [1, 0, 4, 0, 1, 5, 3]]
 }

In order to select all the elements, except the last 3 (of each array)?

Expected output:

        {
           data: [[1, 0, 4, 0], [1, 0, 4, 0]]
        }

whoami - fakeFaceTrueSoul · Accepted Answer · 2020-04-13 17:53:30Z

3

You can use $map to iterate over array's & do the same :

db.collection.aggregate([
  {
    $addFields: {
      data: {
        $map: {
          input: "$data",
          in: {
            "$slice": [
              "$$this",
              0,
              {
                $subtract: [
                  {
                    $size: "$$this"
                  },
                  3
                ]
              }
            ]
          }
        }
      }
    }
  }
])

Test : MongoDB-Playground

answered Apr 13, 2020 at 17:53

whoami - fakeFaceTrueSoul

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1 Comment

William Over a year ago

Thank you very much

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