Concern to get random document(s) from a model via mongoid, see http://stackoverflow.com/questions/7759250/mongoid-random-document/33784640#33784640

randomizable.rb

module Randomizable
  extend ActiveSupport::Concern

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

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

randomizable_spec.rb

require 'spec_helper'

class RandomizableCollection
  include Mongoid::Document
  include Randomizable

  field :name
end

describe RandomizableCollection do
  before do
    RandomizableCollection.create name: 'Hans Bratwurst'
    RandomizableCollection.create name: 'Werner Salami'
    RandomizableCollection.create name: 'Susi Wienerli'
  end

  it 'returns a random document' do
    srand(2)

    expect(RandomizableCollection.random(1).name).to eq 'Werner Salami'
  end

  it 'returns an array of random documents' do
    srand(1)

    expect(RandomizableCollection.random(2).map(&:name)).to eq ['Susi Wienerli', 'Hans Bratwurst']
  end
end