MongoDB Aggregate Second Element from Input Element?

To aggregate second element from input element in MongoDB, use mapReduce() with a counter variable in the scope. This method allows you to skip elements and select specific positions from a collection based on a divisor pattern.

Syntax

db.collection.mapReduce(
    function() {
        track++;
        if (track % div == 0) {
            emit(key, value);
        }
    },
    function() {},
    {
        "scope": { "track": 0, "div": 2 },
        "out": { "inline": 1 }
    }
);

Sample Data

db.demo621.insertMany([
    { _id: 101, Name1: "John", Name2: "John" },
    { _id: 102, Name1: "Bob", Name2: "John" },
    { _id: 103, Name1: "Chris", Name2: "John" },
    { _id: 104, Name1: "Sam", Name2: "John" }
]);

{ "acknowledged": true, "insertedCount": 4 }

Display All Documents

db.demo621.find();

{ "_id": 101, "Name1": "John", "Name2": "John" }
{ "_id": 102, "Name1": "Bob", "Name2": "John" }
{ "_id": 103, "Name1": "Chris", "Name2": "John" }
{ "_id": 104, "Name1": "Sam", "Name2": "John" }

Example: Aggregate Every Second Element

db.demo621.mapReduce(
    function() {
        track++;
        var actualId = this._id;
        delete this._id;
        if (track % div == 0)
            emit(actualId, this);
    },
    function() {},
    {
        "scope": { "track": 0, "div": 2 },
        "out": { "inline": 1 }
    }
);

{
    "results": [
        {
            "_id": 102,
            "value": {
                "Name1": "Bob",
                "Name2": "John"
            }
        },
        {
            "_id": 104,
            "value": {
                "Name1": "Sam",
                "Name2": "John"
            }
        }
    ],
    "timeMillis": 48,
    "counts": {
        "input": 4,
        "emit": 2,
        "reduce": 0,
        "output": 2
    },
    "ok": 1
}

How It Works

• The track variable increments for each document processed
• When track % div == 0 (every 2nd element), the document is emitted
• The scope parameter defines global variables accessible in map function
• Setting div: 2 selects every second element (positions 2, 4, 6, etc.)

Conclusion

MapReduce with scope variables provides a way to select elements at specific intervals from MongoDB collections. By using a counter and modulo operation, you can efficiently extract every nth element from your dataset.