mongo db aggregate randomize ( shuffle ) results

Question

I was going thru bunch of mongo docs and can't find a possibility to shuffle or randomize result content

is there any ?

Answer 1

Specifically for the aggregation framework itself there is not really any native way as there is no available operator as yet to do something like generate a random number. So whatever match you could possibly project a field to sort on would not be "truly random" for lack of a shifting seed value.

The better approach is to "shuffle" the results as an array after the result is returned. There are various "shuffle" implementations, here is one for JavaScript:

function shuffle(array) {
   var currentIndex = array.length
    , temporaryValue
    , randomIndex
    ;

  while (0 !== currentIndex) {
    randomIndex = Math.floor(Math.random() * currentIndex);
    currentIndex -= 1;

    temporaryValue = array[currentIndex];
    array[currentIndex] = array[randomIndex];
    array[randomIndex] = temporaryValue;
  }

  return array;
}

But if you are actually talking about shuffling a large number of results such as in a collection obtained from use of the new $out operator or any collection in fact, then you can "cheat" by using mapReduce.

db.collection.mapReduce(
    function(){
        var random = Math.floor( Math.random() * 100000 );
        emit({ rand: random, id: this._id }, this );
    },
    function(){},
    { out: { replace: "newcollection" } }
);

This takes advantage of the nature of mapReduce in that the key value is always sorted. So by including a random number as the leading part of the key then you will always get a random ordered result.