Mongoid random document

Question

Lets say I have a Collection of users. Is there a way of using mongoid to find n random users in the collection where it does not return the same user twice? For now lets say

Answer 1

Just encountered such a problem. Tried

Model.all.sample

and it works for me

Answer 2

MongoDB 3.2 comes to the rescue with $sample (link to doc)

EDIT : The most recent of Mongoid has implemented $sample, so you can call YourCollection.all.sample(5)

Previous versions of mongoid

Mongoid doesn't support sample until Mongoid 6, so you have to run this aggregate query with the Mongo driver :

samples = User.collection.aggregate([ { '$sample': { size: 3 } } ])
# call samples.to_a if you want to get the objects in memory

What you can do with that

I believe the functionnality should make its way soon to Mongoid, but in the meantime

module Utility
  module_function
  def sample(model, count)
    ids = model.collection.aggregate([ 
      { '$sample': { size: count } }, # Sample from the collection
      { '$project': { _id: 1} }       # Keep only ID fields
    ]).to_a.map(&:values).flatten     # Some Ruby magic

    model.find(ids)
  end
end

Utility.sample(User, 50)

Answer 3

If you really want simplicity you could use this instead:

class Mongoid::Criteria

  def random(n = 1)
    indexes = (0..self.count-1).sort_by{rand}.slice(0,n).collect!

    if n == 1
      return self.skip(indexes.first).first
    else
      return indexes.map{ |index| self.skip(index).first }
    end
  end

end

module Mongoid
  module Finders

    def random(n = 1)
      criteria.random(n)
    end

  end
end

You just have to call User.random(5) and you'll get 5 random users. It'll also work with filtering, so if you want only registered users you can do User.where(:registered => true).random(5).

This will take a while for large collections so I recommend using an alternate method where you would take a random division of the count (e.g.: 25 000 to 30 000) and randomize that range.