gistfile1.rb

gem "rails", "3.2.21"
require "rails/all"

class MyApp < Rails::Application
end

def path_for(opts)
  MyApp.routes.url_helpers.url_for(only_path: true, **opts)
rescue => e
  e
end

MyApp.routes.draw do
  get "/foo", to: "foo#index"
end

p path_for(controller: "foo", action: "index")
  # => "/foo"

p path_for(controller: "foo", action: "doesnt_exist")
  # => #<ActionController::RoutingError: No route matches {:controller=>"foo", :action=>"doesnt_exist"}>

MyApp.routes.draw do
  get "/foo", to: "foo#index"
  get "/nope", to: redirect("/")
end

p path_for(controller: "foo", action: "index")
  # => "/foo"

p path_for(controller: "foo", action: "doesnt_exist")
  # => "/nope?action=doesnt_exist&controller=foo"