MongoDB lists - get every Nth item

Question

I have a Mongodb schema that looks roughly like:

[
  {
    \"name\" : \"name1\",
    \"instances\" : [ 
      {
        \"value\" : 1,
        \"date\" : ISODate         


        

Answer 1

You can use below aggregation:

db.col.aggregate([
    {
        $project: {
            instances: {
                $map: {
                    input: { $range: [ 0, { $size: "$instances" }, N ] },
                    as: "index",
                    in: { $arrayElemAt: [ "$instances", "$$index" ] }
                }
            }
        }
    }
])

$range generates a list of indexes. Third parameter represents non-zero step. For N = 2 it will be [0,2,4,6...], for N = 3 it will return [0,3,6,9...] and so on. Then you can use $map to get correspinding items from instances array.