Random document from a collection in Mongoose

Question

I want to create a Schema.statics.random function that gets me a random element from the collection. I know there is an example for the native MongoDB driver, b

Answer 1

You can use aggregate:

User.aggregate([
    {$match: {gender: "male"}},
    {$sample: {size: 10}}
], function(err, docs) {
    console.log(docs);
});

Or you can use npm package https://www.npmjs.com/package/mongoose-simple-random

User.findRandom({gender: "male"}, {}, {limit: 10}, function(err, results) { 
    console.log(results); // 10 elements
});

Answer 2

For people looking at this in times of async/await, promises etc.:

MySchema.statics.random = async function() {
  const count = await this.count();
  const rand = Math.floor(Math.random() * count);
  const randomDoc = await this.findOne().skip(rand);
  return randomDoc;
};

Answer 3

A shorter and maybe more performant solution
(we don't iterate through the collection once to count and a second time to skip elements, but mongoose might do that behind the scenes):

Use aggregate and $sample:

Model.aggregate([{ $sample: { size: 1 } }])

Answer 4

I found this Mongoose Schema static function in a GitHub Gist, which should achieve what you are after. It counts number of documents in the collection and then returns one document after skipping a random amount.

QuoteSchema.statics.random = function(callback) {
  this.count(function(err, count) {
    if (err) {
      return callback(err);
    }
    var rand = Math.floor(Math.random() * count);
    this.findOne().skip(rand).exec(callback);
  }.bind(this));
};

Source: https://gist.github.com/3453567

NB I modified the code a bit to make it more readable.

Answer 5

I've implemented a plugin for mongoose that does this in a very efficient way using a $near query on two randomly generated coordinates using a 2dsphere index. Check it out here: https://github.com/matomesc/mongoose-random.

Answer 6

If you are not wanting to add "test like" code into your schema, this uses Mongoose queries.

Model.count().exec(function(err, count){

  var random = Math.floor(Math.random() * count);

  Model.findOne().skip(random).exec(
    function (err, result) {

      // result is random 

  });

});