Pagination in a Grape API with Roar.

cursor_helper.rb

module Api
  module V2
    module Util
      module CursorHelpers
        extend ActiveSupport::Concern

        # apply cursor-based pagination to a collection
        # returns a hash:
        #   results: (paginated collection subset)
        #   next: (cursor to the next page)
        def paginate_by_cursor(coll)
          raise 'Both cursor and offset parameters are present, these are mutually exclusive.' if params.key?(:offset) && params.key?(:cursor)
          results = { results: [], next: nil }
          size = (params[:size] || 5).to_i
          if params.key?(:offset)
            skip = params[:offset].to_i
            coll = coll.skip(skip)
          end
          coll.limit(size).scroll(params[:cursor]) do |record, next_cursor|
            results[:results] << record
            results[:next] = next_cursor.to_s
          end
          results[:total_count] = coll.count if params[:total_count] && coll.respond_to?(:count)
          results
        end
      end
    end
  end
end