RandomSelector for ActiveModel via Mongoid

random_selector.rb

module RandomSelector
  class << self
    def included(base)
      base.module_eval do
        def self.random n, conditions=nil
          criteria = conditions.is_a?(Mongoid::Criteria) ? (self.where({}).merge(conditions)) : self.where(conditions||{})
          cc = criteria.count
          mo = [cc-n,2].max
          criteria.skip((rand*cc%mo).to_i).limit(n)
        end
      end
    end
  end
end