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 might like this approach using the $lookup aggregation. And probably the most convenient and fastest way without any aggregation trick.

Create a collection Names with the following schema

[
  { "_id": 1, "name": "name1" },
  { "_id": 2, "name": "name2" }
]

and then Instances collection having the parent id as "nameId"

[
  { "nameId": 1, "value" : 1, "date" : ISODate("2015-03-04T00:00:00.000Z") },
  { "nameId": 1, "value" : 2, "date" : ISODate("2015-04-01T00:00:00.000Z") },
  { "nameId": 1, "value" : 3, "date" : ISODate("2015-03-05T00:00:00.000Z") },
  { "nameId": 2, "value" : 7, "date" : ISODate("2015-03-04T00:00:00.000Z") }, 
  { "nameId": 2, "value" : 8, "date" : ISODate("2015-04-01T00:00:00.000Z") }, 
  { "nameId": 2, "value" : 4, "date" : ISODate("2015-03-05T00:00:00.000Z") }
]

Now with $lookup aggregation 3.6 syntax you can use $sample inside the $lookup pipeline to get the every Nth element randomly.

db.Names.aggregate([
  { "$lookup": {
    "from": Instances.collection.name,
    "let": { "nameId": "$_id" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": ["$nameId", "$$nameId"] }}},
      { "$sample": { "size": N }}
    ],
    "as": "instances"
  }}
])

You can test it here