Add rel="next" & rel="prev" to will_paginate gem (with custom paths)

will_paginate_helper.rb

# config/initializers/will_paginate_helper.rb
# Tested with will_paginate v3.0.7

module WillPaginate
  module ActionView
    class LinkRenderer < ViewHelpers::LinkRenderer
      include ListingHelper
      protected

      def url(page)
        @base_url_params ||= begin
          url_params = merge_get_params(default_url_params)
          url_params[:only_path] = true
          merge_optional_params(url_params)
        end

        url_params = @base_url_params.dup
        add_current_page_param(url_params, page)

        @options[:listings] ? url_for_listing(url_params) : @template.url_for(url_params)
      end
      
      def url_for_listing(url_params)
        # custom listing path generator
        listing_path(options, url_params)
      end
    end

    def pagination_link_tags(collection = nil, options = {}) #:nodoc:
      options, collection = collection, nil if collection.is_a? Hash
      collection ||= infer_collection_from_controller

      options = options.symbolize_keys
      options[:renderer] ||= LinkRenderer

      super(collection, options)
    end
  end

  # Monkey patch for rel prev & next head tags needed for SEO
  # https://github.com/mislav/will_paginate/pull/324
  # https://webmasters.googleblog.com/2011/09/pagination-with-relnext-and-relprev.html
  module ViewHelpers
    class LinkRenderer < LinkRendererBase
      def to_link_tags
        output = []
        link = '<link rel="%s" href="%s" />'
        output << link % ["prev", url(@collection.previous_page)] if @collection.previous_page
        output << link % ["next", url(@collection.next_page)] if @collection.next_page
        output.join("\n")
      end
    end

    def pagination_link_tags(collection, options = {})
      # early exit if there is nothing to render
      return nil unless collection.total_pages > 1

      options = WillPaginate::ViewHelpers.pagination_options.merge(options)

      # get the renderer instance
      renderer = case options[:renderer]
      when nil
        raise ArgumentError, ":renderer not specified"
      when String
        klass = if options[:renderer].respond_to? :constantize then options[:renderer].constantize
          else Object.const_get(options[:renderer]) # poor man's constantize
          end
        klass.new
      when Class then options[:renderer].new
      else options[:renderer]
      end
      # render HTML for pagination
      renderer.prepare collection, options, self
      output = renderer.to_link_tags
      output = output.html_safe if output.respond_to?(:html_safe)
      output
    end
  end
end