Flowdock
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In config/routes.rb you define URL-to-controller mappings, but the reverse is also possible: an URL can be generated from one of your routing definitions. URL generation functionality is centralized in this module.

See ActionDispatch::Routing for general information about routing and routes.rb.

Tip: If you need to generate URLs from your models or some other place, then ActionController::UrlFor is what you’re looking for. Read on for an introduction.

URL generation from parameters

As you may know, some functions, such as ActionController::Base#url_for and ActionView::Helpers::UrlHelper#link_to, can generate URLs given a set of parameters. For example, you’ve probably had the chance to write code like this in one of your views:

<%= link_to('Click here', :controller => 'users',
        :action => 'new', :message => 'Welcome!') %>
# => "/users/new?message=Welcome%21"

link_to, and all other functions that require URL generation functionality, actually use ActionController::UrlFor under the hood. And in particular, they use the ActionController::UrlFor#url_for method. One can generate the same path as the above example by using the following code:

include UrlFor
url_for(:controller => 'users',
        :action => 'new',
        :message => 'Welcome!',
        :only_path => true)
# => "/users/new?message=Welcome%21"

Notice the :only_path => true part. This is because UrlFor has no information about the website hostname that your Rails app is serving. So if you want to include the hostname as well, then you must also pass the :host argument:

include UrlFor
url_for(:controller => 'users',
        :action => 'new',
        :message => 'Welcome!',
        :host => 'www.example.com')
# => "http://www.example.com/users/new?message=Welcome%21"

By default, all controllers and views have access to a special version of url_for, that already knows what the current hostname is. So if you use url_for in your controllers or your views, then you don’t need to explicitly pass the :host argument.

For convenience reasons, mailers provide a shortcut for ActionController::UrlFor#url_for. So within mailers, you only have to type ‘url_for’ instead of ‘ActionController::UrlFor#url_for’ in full. However, mailers don’t have hostname information, and that’s why you’ll still have to specify the :host argument when generating URLs in mailers.

URL generation for named routes

UrlFor also allows one to access methods that have been auto-generated from named routes. For example, suppose that you have a ‘users’ resource in your config/routes.rb:

resources :users

This generates, among other things, the method users_path. By default, this method is accessible from your controllers, views and mailers. If you need to access this auto-generated method from other places (such as a model), then you can do that by including ActionController::UrlFor in your class:

class User < ActiveRecord::Base
  include Rails.application.routes.url_helpers

  def base_uri
    user_path(self)
  end
end

User.find(1).base_uri # => "/users/1"
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