module ActionDispatch::Routing::Mapper::Base

def define_generate_prefix(app, name)
  return unless app.respond_to?(:routes) && app.routes.respond_to?(:define_mounted_helper)
  _route = @set.named_routes.routes[name.to_sym]
  _routes = @set
  app.routes.define_mounted_helper(name)
  app.routes.singleton_class.class_eval do
    define_method :mounted? do
      true
    end
    define_method :_generate_prefix do |options|
      prefix_options = options.slice(*_route.segment_keys)
      # 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