# File lib/dragonfly/data_storage/file_data_store.rb, line 61
      def url_for(relative_path, opts={})
        if server_root.nil?
          raise NotConfigured, "you need to configure server_root for #{self.class.name} in order to form urls"
        else
          _, __, path = absolute(relative_path).partition(server_root)
          if path.empty?
            raise UnableToFormUrl, "couldn't form url for uid #{relative_path.inspect} with root_path #{root_path.inspect} and server_root #{server_root.inspect}"
          else
            path
          end
        end
      end