module Jekyll

  class SortedForTag < Liquid::For

    def render(context)

      sorted_collection = context[@collection_name].dup

      sorted_collection = sorted_collection.sort_by { |i| i.to_liquid[@attributes['sort_by']] }

      sorted_collection_name = "#{@collection_name}_sorted".sub('.', '_')

      context[sorted_collection_name] = sorted_collection

      @collection_name = sorted_collection_name

      super

    end

    def end_tag

      'endsorted_for'

    end

  end

end

Liquid::Template.register_tag('sorted_for', Jekyll::SortedForTag)