define_generate_prefix

# File actionpack/lib/action_dispatch/routing/mapper.rb, line 624
          def define_generate_prefix(app, name)
            _route = @set.named_routes.get name
            _routes = @set
            app.routes.define_mounted_helper(name)
            app.routes.extend Module.new {
              def optimize_routes_generation?; false; end
              define_method :find_script_name do |options|
                if options.key? :script_name
                  super(options)
                else
                  prefix_options = options.slice(*_route.segment_keys)
                  prefix_options[:relative_url_root] = ''.freeze
                  # we must actually delete prefix segment keys to avoid passing them to next url_for
                  _route.segment_keys.each { |k| options.delete(k) }
                  _routes.url_helpers.send("#{name}_path", prefix_options)
                end
              end
            }
          end