def relative_path
      @rel_path ||= begin 
        date_array = id.split("-")[0..2]
        date_path = date_array.join("/")
        article_id = begin
          str = id.gsub date_array.join("-"), ''
          if str[0] == "-"
            str[1..-1] 
          else
            str
          end
        end
        "#{category}/#{date_path}/#{article_id}"
      end
    end
def relative_path
      @rel_path ||= begin 
        date_array = id.split("-")[0..2]
        date_path = date_array.join("/")
        article_id = begin
          str = id.gsub date_array.join("-"), ''
          if str[0] == "-"
            str[1..-1] 
          else
            str
          end
        end
        "#{category}/#{date_path}/#{article_id}"
      end
    end
Return relative path with the Rails.root/public part out.