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, but I can't get it working in Mongoose.

score 41 · Accepted Answer · answered Feb 01 '13 at 11:05

41

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.

answered Feb 01 '13 at 11:05

matthewtole

3,207
22
19

Cool. You know how this could be transalted to CoffeeScript? – user1680104 Feb 01 '13 at 11:13
Okay, it didn't work, so I wasn't sure (that was the problem in first case), but since I still face the problem with inline Javscript. It's rather strange. The stuff inside function(callback) { } is never called. Maybe some problem with the DB-Connection. I'll try to figure it out. – user1680104 Feb 01 '13 at 12:59
Hmm, it's strange. The callback never gets called. – user1680104 Feb 01 '13 at 13:42
I've tested it and it works fine. Could you post your entire schema and I'll see if there's something I can see? – matthewtole Feb 01 '13 at 13:45
Well, it is working. Just somehow connecting to a remote server doesn't work. – user1680104 Feb 01 '13 at 15:41
It all came down to my development machine. So if anyone ever has the same problem: Set up another system. ;) – user1680104 Feb 01 '13 at 18:43
1. Ewww coffee script, 2. they have js to coffee script online converters, just a google away – jemiloii Oct 14 '16 at 23:16

score 23 · Answer 2 · answered Feb 04 '15 at 20:59

23

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 

  });

});

answered Feb 04 '15 at 20:59

Eat at Joes

4,937
1
40
40

I found this really easy to implement – chris31389 Sep 11 '15 at 08:27
How about multiple documents in one query. Is there an easier solution other than querying multiple times? – codersaif May 30 '16 at 07:23
@codersaif This is not what you'd want to use for that. The skip function is directed at pagination and is a cursor, [mongodb cursor](https://docs.mongodb.com/manual/reference/method/cursor.skip/). This will move the cursor to a specific position and is meant to read forward. The code in my answer is basically pagination, `Model.findOne().skip(100).limit(1).exec()` but is fetching a single document. – Eat at Joes Nov 08 '16 at 11:48
3

@PrathameshMore 1)`Model.count()` retrieves how many documents are actually in the collection. 2) `Math....` gets a random whole number within the range of documents counted. 3) Finally we hit mongo one more time with the `findOne()` using the `skip` function to get a single document indexed to the random number in range. – Eat at Joes Sep 23 '19 at 20:49
What if I want to fetch 10 documents from the database? How can I implement this? – Prathamesh More Apr 21 '20 at 16:34

score 17 · Answer 3 · answered Jul 10 '19 at 13:55

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 } }])

score 14 · Answer 4 · answered Jul 31 '19 at 10:31

14

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
});

answered Jul 31 '19 at 10:31

Deepak

3,134
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24

3

this should be marked as the answer – Chukwuemeka Maduekwe Oct 27 '21 at 19:14

score 6 · Answer 5 · answered Jul 03 '13 at 20:50

6

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.

answered Jul 03 '13 at 20:50

Mihai Tomescu

1,525
16
21

score 3 · Answer 6 · answered Apr 10 '18 at 21:21

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;
};

Jakub Siwiec · Answer 7 · 2021-02-03T10:06:57.040

1

Nowadays Model.estimatedDocumentCount() is recommended. For the Model called Item an outline of a function would be:

const Item = require('../your/path/to/Model/Item')

async function getRandomItem() {
   const numItems = await Item.estimatedDocumentCount()
   const rand = Math.floor(Math.random() * numItems)
   const randomItem = await Item.findOne().skip(rand)
   return randomItem
}

edited Feb 03 '21 at 10:06

answered Feb 03 '21 at 09:58

Jakub Siwiec

428
4
13

score 0 · Answer 8 · answered Feb 05 '21 at 00:36

// Getting estimated document count.
yourModel.estimatedDocumentCount().then((count) => {
  //Random number between 0 and count.
  const rand = Math.floor(Math.random() * count);

  // Finding ONE random document.
  yourModel
    .findOne()
    .skip(rand)
    .then((randomDocument) => {
      console.log(randomDocument);
    });
});

You can also use countDocuments(), but estimatedDocumentCount() is recommended for performance.

I prefer this method due to getting one document instead of an array.

score 0 · Answer 9 · answered Feb 15 '21 at 19:03

Get the total number of documents in collection.
Define a random number between 1 and the total number of documents.
Get a document and skip that random number.

const total = await model.countDocuments();
const skip = Math.floor(Math.random() * total) + 1;
const randomDoc = await model.findOne({}).skip(skip).exec();

score 0 · Answer 10 · edited Aug 23 '22 at 12:48

0

command which gives 3 random documents:

db.collection.aggregate([{$sample:{size: 3}}])

express line to get 4 random documents. i think it works in the latest versions of mongo.

db.aggregate([{$sample:{size: 4}}])

see this link for further info

edited Aug 23 '22 at 12:48

Community

1
1

answered Jul 05 '21 at 20:16

Mylu Rohith Bharagav

1
1

Random document from a collection in Mongoose

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