Reverse pagination with kaminari

Question

I want to create pagination for a messaging system in which the first page shown contains the oldest messages, with subsequent pages showing newer messages.

For example, if normal pagination for {a,b,c,d,e,f,g,h,i} with 3 per page is:

{a,b,c}, {d,e,f}, {g,h,i}

Then reverse pagination would be:

{g,h,i}, {d,e,f}, {a,b,c}

I plan to prepend the pages so the result is the same as normal pagination, only starting from the last page.

Is this possible with kaminari?

Answer 1

Kaminary.paginate_array does not produce query with offset and limit. For optimization reason, you shouldn't use this.

Instead you can do this:

@messages = query_for_message.order('created_at DESC').page(params[:page]).per(3)

Where query_for_message stands for any query which you use to retrieve the records for pagination. For example, it can be all the messages of a particular conversation.

Now in the view file, you just need to display @messages in the reverse order. For example:

<%= render :collection => @messages.reverse, :partial => 'message' %>
<%= paginate @messages %>

Answer 2

There's a good example repo on Github called reverse_kaminari on github. It suggests an implementation along these lines (Source).

class CitiesController < ApplicationController

  def index
    @cities = prepare_cities City.order('created_at DESC')
  end

  private

  def prepare_cities(scope)
    @per_page = City.default_per_page
    total_count = scope.count
    rest_count = total_count > @per_page ? (total_count % @per_page) : 0
    @num_pages = total_count > @per_page ? (total_count / @per_page) : 1

    if params[:page]
      offset = params[:page].sub(/-.*/, '').to_i
      current_page = @num_pages - (offset - 1) / @per_page
      scope.page(current_page).per(@per_page).padding(rest_count)
    else
      scope.page(1).per(@per_page + rest_count)
    end
  end

end

All credits go to Andrew Djoga. He also hosted the app as a working demo.