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

Unfortunately, with the aggregation framework it's not possible as this would require an option with $unwind to emit an array index/position, of which currently aggregation can't handle. There is an open JIRA ticket for this here SERVER-4588.

However, a workaround would be to use MapReduce but this comes at a huge performance cost since the actual calculations of getting the array index are performed using the embedded JavaScript engine (which is slow), and there still is a single global JavaScript lock, which only allows a single JavaScript thread to run at a single time.

With mapReduce, you could try something like this:

Mapping function:

var map = function(){
    for(var i=0; i < this.instances.length; i++){
        emit(
            { "_id": this._id,  "index": i },
            { "index": i, "value": this.instances[i] }
        );
    }
};

Reduce function:

var reduce = function(){}

You can then run the following mapReduce function on your collection:

db.collection.mapReduce( map, reduce, { out : "resultCollection" } );

And then you can query the result collection to geta list/array of every Nth item of the instance array by using the map() cursor method :

var thirdInstances = db.resultCollection.find({"_id.index": N})
                                        .map(function(doc){return doc.value.value})