So, I hacked together something for Grape to give me opaque cursors for pagination with Kaminari. It works, but could be a lot nicer. For instance, it doesn't handle query parameters at the moment.

pagination.rb

require 'msgpack'
require "kaminari/grape"

module API
  module V1
    module Pagination
      def self.included(base)
        base.class_eval do
          helpers do
            def paginate(collection)
              options = {
                page: 0,
                per_page: ::Kaminari.config.default_per_page || 100,
              }

              if params.has_key?(:cursor)
                cursor = API::V1::Pagination.decode_cursor(params[:cursor])
                options.merge!(cursor)
              end



              collection.page(options[:page]).per(options[:per_page]).tap do |data|
                header "next_cursor", API::V1::Pagination.encode_cursor(page: data.next_page, per_page: options[:per_page]) unless data.last_page?
                header "prev_cursor", API::V1::Pagination.encode_cursor(page: data.prev_page, per_page: options[:per_page]) unless data.first_page?
              end
            end
          end

          def self.paginate(options = {})
            params do
              optional :cursor, type: String, desc: 'Cursor for paging results.'
            end
          end
        end
      end

      class << self
        def decode_cursor(cursor)
          data = Base64.urlsafe_decode64(cursor + '=' * (offset = 4 - cursor.size.modulo(4) and offset == 4 ? 0 : offset))
          data = MessagePack.unpack(data, symbolize_keys: true)
          data
        end

        def encode_cursor(options)
          cursor = MessagePack.pack(options)
          cursor = Base64.urlsafe_encode64(cursor).gsub('=', '')
          cursor
        end
      end
    end
  end
end

Oh, yeah, using JSON here really isn't right, nor is using Base64 and then removing the padding nice either. However, on Base64, what I really wanted was Base62, but all the Base62 gems do nasty shit like monkey-patching things.

It'd probably be better to use MSGPack or something, maybe.

i want to implement cursor based pagination, how would i drop in this class, and use it? @miksago