Paginating a shuffled ActiveRecord query

Question

I am attempting to paginate a shuffled ActiveRecord query. The syntax for doing this using the Kaminari gem is:

@users = Kaminari.paginate_array(User.all.shuffle).page(params[:page]).per(20)

The issue with this is that User.all is re-shuffled on each pagination request, causing duplicate records to be called. Is there any way to prevent this kind of duplication?

Answer 1

You need to pass seed for rand between queries

params[:seed] ||= Random.new_seed
srand params[:seed].to_i
@users = Kaminari.paginate_array(User.all.shuffle).page(params[:page]).per(20)

And in view add params[:seed] to all kaminari links to pages

Answer 2

As KandadaBoggu points out above, retrieving all of the User records from the database is inefficient when you only need 20. I would suggest using MySQL's RAND() function to perform the randomization before you return from the database. You can still pass a seed value to RAND() to make sure the shuffling only happens once per session.

For example:

class User < ActiveRecord::Base
  def self.randomized(seed = nil)
    seed = seed.to_i rescue 0
    order("RAND(#{seed})")
  end
end

class UsersController < ApplicationController
  before_filter :set_random_seed

  def index
    @users = User.randomized(session[:seed]).page(params[:page]).per(20)
  end

private

  def set_random_seed
    session[:seed] ||= Random.new_seed
  end
end

I don't have a MySQL installation to test against, but this should perform better than your original code.