Action Controllers are the core of a web request in Rails. They are made up of one or more actions that are executed on request and then either render a template or redirect to another action. An action is defined as a public method on the controller, which will automatically be made accessible to the web-server through Rails Routes.

A sample controller could look like this:

  class GuestBookController < ActionController::Base
    def index
      @entries = Entry.find(:all)

    def sign
      redirect_to :action => "index"

Actions, by default, render a template in the app/views directory corresponding to the name of the controller and action after executing code in the action. For example, the index action of the GuestBookController would render the template app/views/guestbook/index.erb by default after populating the @entries instance variable.

Unlike index, the sign action will not render a template. After performing its main purpose (creating a new entry in the guest book), it initiates a redirect instead. This redirect works by returning an external "302 Moved" HTTP response that takes the user to the index action.

The index and sign represent the two basic action archetypes used in Action Controllers. Get-and-show and do-and-redirect. Most actions are variations of these themes.


Requests are processed by the Action Controller framework by extracting the value of the "action" key in the request parameters. This value should hold the name of the action to be performed. Once the action has been identified, the remaining request parameters, the session (if one is available), and the full request with all the HTTP headers are made available to the action through instance variables. Then the action is performed.

The full request object is available with the request accessor and is primarily used to query for HTTP headers. These queries are made by accessing the environment hash, like this:

  def server_ip
    location = request.env["SERVER_ADDR"]
    render :text => "This server hosted at #{location}"


All request parameters, whether they come from a GET or POST request, or from the URL, are available through the params method which returns a hash. For example, an action that was performed through /weblog/list?category=All&limit=5 will include { "category" => "All", "limit" => 5 } in params.

It‘s also possible to construct multi-dimensional parameter hashes by specifying keys using brackets, such as:

  <input type="text" name="post[name]" value="david">
  <input type="text" name="post[address]" value="hyacintvej">

A request stemming from a form holding these inputs will include { "post" => { "name" => "david", "address" => "hyacintvej" } }. If the address input had been named "post[address][street]", the params would have included { "post" => { "address" => { "street" => "hyacintvej" } } }. There‘s no limit to the depth of the nesting.


Sessions allows you to store objects in between requests. This is useful for objects that are not yet ready to be persisted, such as a Signup object constructed in a multi-paged process, or objects that don‘t change much and are needed all the time, such as a User object for a system that requires login. The session should not be used, however, as a cache for objects where it‘s likely they could be changed unknowingly. It‘s usually too much work to keep it all synchronized — something databases already excel at.

You can place objects in the session by using the session method, which accesses a hash:

  session[:person] = Person.authenticate(user_name, password)

And retrieved again through the same hash:

  Hello #{session[:person]}

For removing objects from the session, you can either assign a single key to nil:

  # removes :person from session
  session[:person] = nil

or you can remove the entire session with reset_session.

Sessions are stored by default in a browser cookie that‘s cryptographically signed, but unencrypted. This prevents the user from tampering with the session but also allows him to see its contents.

Do not put secret information in cookie-based sessions!

Other options for session storage are:

  • ActiveRecord::SessionStore - Sessions are stored in your database, which works better than PStore with multiple app servers and, unlike CookieStore, hides your session contents from the user. To use ActiveRecord::SessionStore, set
      config.action_controller.session_store = :active_record_store

    in your config/environment.rb and run rake db:sessions:create.

  • MemCacheStore - Sessions are stored as entries in your memcached cache. Set the session store type in config/environment.rb:
      config.action_controller.session_store = :mem_cache_store

    This assumes that memcached has been installed and configured properly. See the MemCacheStore docs for more information.


Each action results in a response, which holds the headers and document to be sent to the user‘s browser. The actual response object is generated automatically through the use of renders and redirects and requires no user intervention.


Action Controller sends content to the user by using one of five rendering methods. The most versatile and common is the rendering of a template. Included in the Action Pack is the Action View, which enables rendering of ERb templates. It‘s automatically configured. The controller passes objects to the view by assigning instance variables:

  def show
    @post = Post.find(params[:id])

Which are then automatically available to the view:

  Title: <%= @post.title %>

You don‘t have to rely on the automated rendering. Especially actions that could result in the rendering of different templates will use the manual rendering methods:

  def search
    @results = Search.find(params[:query])
    case @results
      when 0 then render :action => "no_results"
      when 1 then render :action => "show"
      when 2..10 then render :action => "show_many"

Read more about writing ERb and Builder templates in classes/ActionView/Base.html.


Redirects are used to move from one action to another. For example, after a create action, which stores a blog entry to a database, we might like to show the user the new entry. Because we‘re following good DRY principles (Don‘t Repeat Yourself), we‘re going to reuse (and redirect to) a show action that we‘ll assume has already been created. The code might look like this:

  def create
    @entry =[:entry])
      # The entry was saved correctly, redirect to show
      redirect_to :action => 'show', :id =>
      # things didn't go so well, do something else

In this case, after saving our new entry to the database, the user is redirected to the show method which is then executed.

Calling multiple redirects or renders

An action may contain only a single render or a single redirect. Attempting to try to do either again will result in a DoubleRenderError:

  def do_something
    redirect_to :action => "elsewhere"
    render :action => "overthere" # raises DoubleRenderError

If you need to redirect on the condition of something, then be sure to add "and return" to halt execution.

  def do_something
    redirect_to(:action => "elsewhere") and return if monkeys.nil?
    render :action => "overthere" # won't be called if monkeys is nil
Included Modules
[RW] action_name Returns the name of the action this controller is processing.
Public Class methods

Adds a view_path to the end of the view_paths array. If the current class has no view paths, copy them from the superclass. This change will be visible for all future requests.

  ArticleController.append_view_path(["views/default", "views/custom"])
     # File actionpack/lib/action_controller/base.rb, line 455
455:       def append_view_path(path)
456:         @view_paths = superclass.view_paths.dup if @view_paths.nil?
457:         @view_paths.push(*path)
458:       end
     # File actionpack/lib/action_controller/base.rb, line 382
382:       def call(env)
383:         # HACK: For global rescue to have access to the original request and response
384:         request = env["action_controller.rescue.request"] ||=
385:         response = env["action_controller.rescue.response"] ||=
386:         process(request, response)
387:       end

Converts the class name from something like "OneModule::TwoModule::NeatController" to "NeatController".

     # File actionpack/lib/action_controller/base.rb, line 395
395:       def controller_class_name
396:         @controller_class_name ||= name.demodulize
397:       end

Converts the class name from something like "OneModule::TwoModule::NeatController" to "neat".

     # File actionpack/lib/action_controller/base.rb, line 400
400:       def controller_name
401:         @controller_name ||= controller_class_name.sub(/Controller$/, '').underscore
402:       end

Converts the class name from something like "OneModule::TwoModule::NeatController" to "one_module/two_module/neat".

     # File actionpack/lib/action_controller/base.rb, line 405
405:       def controller_path
406:         @controller_path ||= name.gsub(/Controller$/, '').underscore
407:       end
filter_parameter_logging(*filter_words) {|key, value| ...}

Replace sensitive parameter data from the request log. Filters parameters that have any of the arguments as a substring. Looks in all subhashes of the param hash for keys to filter. If a block is given, each key and value of the parameter hash and all subhashes is passed to it, the value or key can be replaced using String#replace or similar method.


  => Does nothing, just slows the logging process down

  filter_parameter_logging :password
  => replaces the value to all keys matching /password/i with "[FILTERED]"

  filter_parameter_logging :foo, "bar"
  => replaces the value to all keys matching /foo|bar/i with "[FILTERED]"

  filter_parameter_logging { |k,v| v.reverse! if k =~ /secret/i }
  => reverses the value to all keys matching /secret/i

  filter_parameter_logging(:foo, "bar") { |k,v| v.reverse! if k =~ /secret/i }
  => reverses the value to all keys matching /secret/i, and
     replaces the value to all keys matching /foo|bar/i with "[FILTERED]"
     # File actionpack/lib/action_controller/base.rb, line 483
483:       def filter_parameter_logging(*filter_words, &block)
484:         parameter_filter ={ |s| s.to_s }.join('|'), true) if filter_words.length > 0
486:         define_method(:filter_parameters) do |unfiltered_parameters|
487:           filtered_parameters = {}
489:           unfiltered_parameters.each do |key, value|
490:             if key =~ parameter_filter
491:               filtered_parameters[key] = '[FILTERED]'
492:             elsif value.is_a?(Hash)
493:               filtered_parameters[key] = filter_parameters(value)
494:             elsif value.is_a?(Array)
495:               filtered_parameters[key] = value.collect do |item|
496:                 case item
497:                 when Hash, Array
498:                   filter_parameters(item)
499:                 else
500:                   item
501:                 end
502:               end
503:             elsif block_given?
504:               key = key.dup
505:               value = value.dup if value.duplicable?
506:               yield key, value
507:               filtered_parameters[key] = value
508:             else
509:               filtered_parameters[key] = value
510:             end
511:           end
513:           filtered_parameters
514:         end
515:         protected :filter_parameters
516:       end

Return an array containing the names of public methods that have been marked hidden from the action processor. By default, all methods defined in ActionController::Base and included modules are hidden. More methods can be hidden using hide_actions.

     # File actionpack/lib/action_controller/base.rb, line 412
412:       def hidden_actions
413:         read_inheritable_attribute(:hidden_actions) || write_inheritable_attribute(:hidden_actions, [])
414:       end

Hide each of the given methods from being callable as actions.

     # File actionpack/lib/action_controller/base.rb, line 417
417:       def hide_action(*names)
418:         write_inheritable_attribute(:hidden_actions, hidden_actions | { |name| name.to_s })
419:       end

Adds a view_path to the front of the view_paths array. If the current class has no view paths, copy them from the superclass. This change will be visible for all future requests.

  ArticleController.prepend_view_path(["views/default", "views/custom"])
     # File actionpack/lib/action_controller/base.rb, line 443
443:       def prepend_view_path(path)
444:         @view_paths = superclass.view_paths.dup if !defined?(@view_paths) || @view_paths.nil?
445:         @view_paths.unshift(*path)
446:       end

View load paths determine the bases from which template references can be made. So a call to render("test/template") will be looked up in the view load paths array and the closest match will be returned.

     # File actionpack/lib/action_controller/base.rb, line 424
424:       def view_paths
425:         if defined? @view_paths
426:           @view_paths
427:         else
428:           superclass.view_paths
429:         end
430:       end
     # File actionpack/lib/action_controller/base.rb, line 432
432:       def view_paths=(value)
433:         @view_paths = ActionView::Base.process_view_paths(value) if value
434:       end
Public Instance methods

Adds a view_path to the end of the view_paths array. This change affects the current request only.

  self.append_view_path(["views/default", "views/custom"])
     # File actionpack/lib/action_controller/base.rb, line 677
677:       def append_view_path(path)
678:         @template.view_paths.push(*path)
679:       end

Converts the class name from something like "OneModule::TwoModule::NeatController" to "NeatController".

     # File actionpack/lib/action_controller/base.rb, line 632
632:       def controller_class_name
633:         self.class.controller_class_name
634:       end

Converts the class name from something like "OneModule::TwoModule::NeatController" to "neat".

     # File actionpack/lib/action_controller/base.rb, line 637
637:       def controller_name
638:         self.class.controller_name
639:       end

Converts the class name from something like "OneModule::TwoModule::NeatController" to "one_module/two_module/neat".

     # File actionpack/lib/action_controller/base.rb, line 642
642:       def controller_path
643:         self.class.controller_path
644:       end

Adds a view_path to the front of the view_paths array. This change affects the current request only.

  self.prepend_view_path(["views/default", "views/custom"])
     # File actionpack/lib/action_controller/base.rb, line 667
667:       def prepend_view_path(path)
668:         @template.view_paths.unshift(*path)
669:       end
     # File actionpack/lib/action_controller/base.rb, line 539
539:       def send_response
540:         response.prepare!
541:         response
542:       end
     # File actionpack/lib/action_controller/base.rb, line 646
646:       def session_enabled?
647:         ActiveSupport::Deprecation.warn("Sessions are now lazy loaded. So if you don't access them, consider them disabled.", caller)
648:       end
url_for(options = {})

Returns a URL that has been rewritten according to the options hash and the defined routes. (For doing a complete redirect, use redirect_to).

url_for is used to:

All keys given to url_for are forwarded to the Route module, save for the following:

  • :anchor - Specifies the anchor name to be appended to the path. For example, url_for :controller => ‘posts’, :action => ‘show’, :id => 10, :anchor => ‘comments‘ will produce "/posts/show/10#comments".
  • :only_path - If true, returns the relative URL (omitting the protocol, host name, and port) (false by default).
  • :trailing_slash - If true, adds a trailing slash, as in "/archive/2005/". Note that this is currently not recommended since it breaks caching.
  • :host - Overrides the default (current) host if provided.
  • :protocol - Overrides the default (current) protocol if provided.
  • :port - Optionally specify the port to connect to.
  • :user - Inline HTTP authentication (only plucked out if :password is also present).
  • :password - Inline HTTP authentication (only plucked out if :user is also present).
  • :skip_relative_url_root - If true, the url is not constructed using the relative_url_root of the request so the path will include the web server relative installation directory.

The URL is generated from the remaining keys in the hash. A URL contains two key parts: the <base> and a query string. Routes composes a query string as the key/value pairs not included in the <base>.

The default Routes setup supports a typical Rails path of "controller/action/id" where action and id are optional, with action defaulting to ‘index’ when not given. Here are some typical url_for statements and their corresponding URLs:

  url_for :controller => 'posts', :action => 'recent'                # => 'proto://'
  url_for :controller => 'posts', :action => 'index'                 # => 'proto://'
  url_for :controller => 'posts', :action => 'index', :port=>'8033'  # => 'proto://'
  url_for :controller => 'posts', :action => 'show', :id => 10       # => 'proto://'
  url_for :controller => 'posts', :user => 'd', :password => '123'   # => 'proto://'

When generating a new URL, missing values may be filled in from the current request‘s parameters. For example, url_for :action => ‘some_action‘ will retain the current controller, as expected. This behavior extends to other parameters, including :controller, :id, and any other parameters that are placed into a Route‘s path.   The URL helpers such as url_for have a limited form of memory: when generating a new URL, they can look for missing values in the current request‘s parameters. Routes attempts to guess when a value should and should not be taken from the defaults. There are a few simple rules on how this is performed:

  • If the controller name begins with a slash no defaults are used:
      url_for :controller => '/home'

    In particular, a leading slash ensures no namespace is assumed. Thus, while url_for :controller => ‘users‘ may resolve to Admin::UsersController if the current controller lives under that module, url_for :controller => ’/users‘ ensures you link to ::UsersController no matter what.

  • If the controller changes, the action will default to index unless provided

The final rule is applied while the URL is being generated and is best illustrated by an example. Let us consider the route given by map.connect ‘people/:last/:first/:action’, :action => ‘bio’, :controller => ‘people‘.

Suppose that the current URL is "people/hh/david/contacts". Let‘s consider a few different cases of URLs which are generated from this page.

  • url_for :action => ‘bio‘ — During the generation of this URL, default values will be used for the first and

last components, and the action shall change. The generated URL will be, "people/hh/david/bio".

  • url_for :first => ‘davids-little-brother‘ This generates the URL ‘people/hh/davids-little-brother’ — note that this URL leaves out the assumed action of ‘bio’.

However, you might ask why the action from the current request, ‘contacts’, isn‘t carried over into the new URL. The answer has to do with the order in which the parameters appear in the generated path. In a nutshell, since the value that appears in the slot for :first is not equal to default value for :first we stop using defaults. On its own, this rule can account for much of the typical Rails URL behavior.   Although a convenience, defaults can occasionally get in your way. In some cases a default persists longer than desired. The default may be cleared by adding :name => nil to url_for‘s options. This is often required when writing form helpers, since the defaults in play may vary greatly depending upon where the helper is used from. The following line will redirect to PostController‘s default action, regardless of the page it is displayed on:

  url_for :controller => 'posts', :action => nil
     # File actionpack/lib/action_controller/base.rb, line 619
619:       def url_for(options = {})
620:         options ||= {}
621:         case options
622:           when String
623:             options
624:           when Hash
625:             @url.rewrite(rewrite_options(options))
626:           else
627:             polymorphic_url(options)
628:         end
629:       end

View load paths for controller.

     # File actionpack/lib/action_controller/base.rb, line 653
653:       def view_paths
654:         @template.view_paths
655:       end
     # File actionpack/lib/action_controller/base.rb, line 657
657:       def view_paths=(value)
658:         @template.view_paths = ActionView::Base.process_view_paths(value)
659:       end
Protected Instance methods
default_url_options(options = nil)

Overwrite to implement a number of default options that all url_for-based methods will use. The default options should come in the form of a hash, just like the one you would use for url_for directly. Example:

  def default_url_options(options)
    { :project => ? @project.url_name : "unknown" }

As you can infer from the example, this is mostly useful for situations where you want to centralize dynamic decisions about the urls as they stem from the business domain. Please note that any individual url_for call can always override the defaults set by this method.

      # File actionpack/lib/action_controller/base.rb, line 1063
1063:       def default_url_options(options = nil)
1064:       end
expires_in(seconds, options = {})

Sets a HTTP 1.1 Cache-Control header. Defaults to issuing a "private" instruction, so that intermediate caches shouldn‘t cache the response.


  expires_in 20.minutes
  expires_in 3.hours, :public => true
  expires in 3.hours, 'max-stale' => 5.hours, :public => true

This method will overwrite an existing Cache-Control header. See for more possibilities.

      # File actionpack/lib/action_controller/base.rb, line 1216
1216:       def expires_in(seconds, options = {}) #:doc:
1217:         cache_control = response.headers["Cache-Control"].split(",").map {|k| k.strip }
1219:         cache_control << "max-age=#{seconds}"
1220:         cache_control.delete("no-cache")
1221:         if options[:public]
1222:           cache_control.delete("private")
1223:           cache_control << "public"
1224:         else
1225:           cache_control << "private"
1226:         end
1228:         # This allows for additional headers to be passed through like 'max-stale' => 5.hours
1229:         cache_control += options.symbolize_keys.reject{|k,v| k == :public || k == :private }.map{ |k,v| v == true ? k.to_s : "#{k.to_s}=#{v.to_s}"}
1231:         response.headers["Cache-Control"] = cache_control.join(', ')
1232:       end

Sets a HTTP 1.1 Cache-Control header of "no-cache" so no caching should occur by the browser or intermediate caches (like caching proxy servers).

      # File actionpack/lib/action_controller/base.rb, line 1236
1236:       def expires_now #:doc:
1237:         response.headers["Cache-Control"] = "no-cache"
1238:       end

Sets the etag, last_modified, or both on the response and renders a "304 Not Modified" response if the request is already fresh.


  • :etag
  • :last_modified
  • :public By default the Cache-Control header is private, set this to true if you want your application to be cachable by other devices (proxy caches).


  def show
    @article = Article.find(params[:id])
    fresh_when(:etag => @article, :last_modified => @article.created_at.utc, :public => true)

This will render the show template if the request isn‘t sending a matching etag or If-Modified-Since header and just a "304 Not Modified" response if there‘s a match.

      # File actionpack/lib/action_controller/base.rb, line 1187
1187:       def fresh_when(options)
1188:         options.assert_valid_keys(:etag, :last_modified, :public)
1190:         response.etag          = options[:etag]          if options[:etag]
1191:         response.last_modified = options[:last_modified] if options[:last_modified]
1193:         if options[:public] 
1194:           cache_control = response.headers["Cache-Control"].split(",").map {|k| k.strip }
1195:           cache_control.delete("private")
1196:           cache_control.delete("no-cache")
1197:           cache_control << "public"
1198:           response.headers["Cache-Control"] = cache_control.join(', ')
1199:         end
1201:         if request.fresh?(response)
1202:           head :not_modified
1203:         end
1204:       end

Return a response that has no content (merely headers). The options argument is interpreted to be a hash of header names and values. This allows you to easily return a response that consists only of significant headers:

  head :created, :location => person_path(@person)

It can also be used to return exceptional conditions:

  return head(:method_not_allowed) unless
  return head(:bad_request) unless valid_request?
      # File actionpack/lib/action_controller/base.rb, line 1005
1005:       def head(*args)
1006:         if args.length > 2
1007:           raise ArgumentError, "too many arguments to head"
1008:         elsif args.empty?
1009:           raise ArgumentError, "too few arguments to head"
1010:         end
1011:         options = args.extract_options!
1012:         status = interpret_status(args.shift || options.delete(:status) || :ok)
1014:         options.each do |key, value|
1015:           headers[key.to_s.dasherize.split(/-/).map { |v| v.capitalize }.join("-")] = value.to_s
1016:         end
1018:         render :nothing => true, :status => status
1019:       end
redirect_to(options = {}, response_status = {})

Redirects the browser to the target specified in options. This parameter can take one of three forms:

  • Hash - The URL will be generated by calling url_for with the options.
  • Record - The URL will be generated by calling url_for with the options, which will reference a named URL for that record.
  • String starting with protocol:// (like http://) - Is passed straight through as the target for redirection.
  • String not containing a protocol - The current protocol and host is prepended to the string.
  • :back - Back to the page that issued the request. Useful for forms that are triggered from multiple places. Short-hand for redirect_to(request.env["HTTP_REFERER"])


  redirect_to :action => "show", :id => 5
  redirect_to post
  redirect_to ""
  redirect_to "/images/screenshot.jpg"
  redirect_to articles_url
  redirect_to :back

The redirection happens as a "302 Moved" header unless otherwise specified.


  redirect_to post_url(@post), :status => :found
  redirect_to :action=>'atom', :status => :moved_permanently
  redirect_to post_url(@post), :status => 301
  redirect_to :action=>'atom', :status => 302

It is also possible to assign a flash message as part of the redirection. There are two special accessors for commonly used the flash names alert and notice as well as a general purpose flash bucket.


  redirect_to post_url(@post), :alert => "Watch it, mister!"
  redirect_to post_url(@post), :status=> :found, :notice => "Pay attention to the road"
  redirect_to post_url(@post), :status => 301, :flash => { :updated_post_id => }
  redirect_to { :action=>'atom' }, :alert => "Something serious happened"

When using redirect_to :back, if there is no referrer, RedirectBackError will be raised. You may specify some fallback behavior for this case by rescuing RedirectBackError.

      # File actionpack/lib/action_controller/base.rb, line 1103
1103:       def redirect_to(options = {}, response_status = {}) #:doc:
1104:         raise"Cannot redirect to nil!") if options.nil?
1106:         if options.is_a?(Hash) && options[:status]
1107:           status = options.delete(:status)
1108:         elsif response_status[:status]
1109:           status = response_status[:status]
1110:         else
1111:           status = 302
1112:         end
1114:         response.redirected_to = options
1116:         case options
1117:           # The scheme name consist of a letter followed by any combination of
1118:           # letters, digits, and the plus ("+"), period ("."), or hyphen ("-")
1119:           # characters; and is terminated by a colon (":").
1120:           when %r{^\w[\w\d+.-]*:.*}
1121:             redirect_to_full_url(options, status)
1122:           when String
1123:             redirect_to_full_url(request.protocol + request.host_with_port + options, status)
1124:           when :back
1125:             if referer = request.headers["Referer"]
1126:               redirect_to(referer, :status=>status)
1127:             else
1128:               raise RedirectBackError
1129:             end
1130:           else
1131:             redirect_to_full_url(url_for(options), status)
1132:         end
1133:       end
redirect_to_full_url(url, status)
      # File actionpack/lib/action_controller/base.rb, line 1135
1135:       def redirect_to_full_url(url, status)
1136:         raise DoubleRenderError if performed?
1137:"Redirected to #{url}") if logger &&
1138:         response.redirect(url, interpret_status(status))
1139:         @performed_redirect = true
1140:       end
render(options = nil, extra_options = {}, &block)

Renders the content that will be returned to the browser as the response body.

Rendering an action

Action rendering is the most common form and the type used automatically by Action Controller when nothing else is specified. By default, actions are rendered within the current layout (if one exists).

  # Renders the template for the action "goal" within the current controller
  render :action => "goal"

  # Renders the template for the action "short_goal" within the current controller,
  # but without the current active layout
  render :action => "short_goal", :layout => false

  # Renders the template for the action "long_goal" within the current controller,
  # but with a custom layout
  render :action => "long_goal", :layout => "spectacular"

Rendering partials

Partial rendering in a controller is most commonly used together with Ajax calls that only update one or a few elements on a page without reloading. Rendering of partials from the controller makes it possible to use the same partial template in both the full-page rendering (by calling it from within the template) and when sub-page updates happen (from the controller action responding to Ajax calls). By default, the current layout is not used.

  # Renders the same partial with a local variable.
  render :partial => "person", :locals => { :name => "david" }

  # Renders the partial, making @new_person available through
  # the local variable 'person'
  render :partial => "person", :object => @new_person

  # Renders a collection of the same partial by making each element
  # of @winners available through the local variable "person" as it
  # builds the complete response.
  render :partial => "person", :collection => @winners

  # Renders a collection of partials but with a custom local variable name
  render :partial => "admin_person", :collection => @winners, :as => :person

  # Renders the same collection of partials, but also renders the
  # person_divider partial between each person partial.
  render :partial => "person", :collection => @winners, :spacer_template => "person_divider"

  # Renders a collection of partials located in a view subfolder
  # outside of our current controller.  In this example we will be
  # rendering app/views/shared/_note.r(html|xml)  Inside the partial
  # each element of @new_notes is available as the local var "note".
  render :partial => "shared/note", :collection => @new_notes

  # Renders the partial with a status code of 500 (internal error).
  render :partial => "broken", :status => 500

Note that the partial filename must also be a valid Ruby variable name, so e.g. 2005 and register-user are invalid.

Automatic etagging

Rendering will automatically insert the etag header on 200 OK responses. The etag is calculated using MD5 of the response body. If a request comes in that has a matching etag, the response will be changed to a 304 Not Modified and the response body will be set to an empty string. No etag header will be inserted if it‘s already set.

Rendering a template

Template rendering works just like action rendering except that it takes a path relative to the template root. The current layout is automatically applied.

  # Renders the template located in [TEMPLATE_ROOT]/weblog/show.r(html|xml) (in Rails, app/views/weblog/show.erb)
  render :template => "weblog/show"

  # Renders the template with a local variable
  render :template => "weblog/show", :locals => {:customer =>}

Rendering a file

File rendering works just like action rendering except that it takes a filesystem path. By default, the path is assumed to be absolute, and the current layout is not applied.

  # Renders the template located at the absolute filesystem path
  render :file => "/path/to/some/template.erb"
  render :file => "c:/path/to/some/template.erb"

  # Renders a template within the current layout, and with a 404 status code
  render :file => "/path/to/some/template.erb", :layout => true, :status => 404
  render :file => "c:/path/to/some/template.erb", :layout => true, :status => 404

Rendering text

Rendering of text is usually used for tests or for rendering prepared content, such as a cache. By default, text rendering is not done within the active layout.

  # Renders the clear text "hello world" with status code 200
  render :text => "hello world!"

  # Renders the clear text "Explosion!"  with status code 500
  render :text => "Explosion!", :status => 500

  # Renders the clear text "Hi there!" within the current active layout (if one exists)
  render :text => "Hi there!", :layout => true

  # Renders the clear text "Hi there!" within the layout
  # placed in "app/views/layouts/special.r(html|xml)"
  render :text => "Hi there!", :layout => "special"

Streaming data and/or controlling the page generation

The :text option can also accept a Proc object, which can be used to:

  1. stream on-the-fly generated data to the browser. Note that you should use the methods provided by ActionController::Steaming instead if you want to stream a buffer or a file.
  2. manually control the page generation. This should generally be avoided, as it violates the separation between code and content, and because almost everything that can be done with this method can also be done more cleanly using one of the other rendering methods, most notably templates.

Two arguments are passed to the proc, a response object and an output object. The response object is equivalent to the return value of the ActionController::Base#response method, and can be used to control various things in the HTTP response, such as setting the Content-Type header. The output object is an writable IO-like object, so one can call write and flush on it.

The following example demonstrates how one can stream a large amount of on-the-fly generated data to the browser:

  # Streams about 180 MB of generated data to the browser.
  render :text => proc { |response, output|
    10_000_000.times do |i|
      output.write("This is line #{i}\n")

Another example:

  # Renders "Hello from code!"
  render :text => proc { |response, output| output.write("Hello from code!") }

Rendering XML

Rendering XML sets the content type to application/xml.

  # Renders '<name>David</name>'
  render :xml => {:name => "David"}.to_xml

It‘s not necessary to call to_xml on the object you want to render, since render will automatically do that for you:

  # Also renders '<name>David</name>'
  render :xml => {:name => "David"}

Rendering JSON

Rendering JSON sets the content type to application/json and optionally wraps the JSON in a callback. It is expected that the response will be parsed (or eval‘d) for use as a data structure.

  # Renders '{"name": "David"}'
  render :json => {:name => "David"}.to_json

It‘s not necessary to call to_json on the object you want to render, since render will automatically do that for you:

  # Also renders '{"name": "David"}'
  render :json => {:name => "David"}

Sometimes the result isn‘t handled directly by a script (such as when the request comes from a SCRIPT tag), so the :callback option is provided for these cases.

  # Renders 'show({"name": "David"})'
  render :json => {:name => "David"}.to_json, :callback => 'show'

Rendering an inline template

Rendering of an inline template works as a cross between text and action rendering where the source for the template is supplied inline, like text, but its interpreted with ERb or Builder, like action. By default, ERb is used for rendering and the current layout is not used.

  # Renders "hello, hello, hello, again"
  render :inline => "<%= 'hello, ' * 3 + 'again' %>"

  # Renders "<p>Good seeing you!</p>" using Builder
  render :inline => "xml.p { 'Good seeing you!' }", :type => :builder

  # Renders "hello david"
  render :inline => "<%= 'hello ' + name %>", :locals => { :name => "david" }

Rendering inline JavaScriptGenerator page updates

In addition to rendering JavaScriptGenerator page updates with Ajax in RJS templates (see ActionView::Base for details), you can also pass the :update parameter to render, along with a block, to render page updates inline.

  render :update do |page|
    page.replace_html  'user_list', :partial => 'user', :collection => @users
    page.visual_effect :highlight, 'user_list'

Rendering vanilla JavaScript

In addition to using RJS with render :update, you can also just render vanilla JavaScript with :js.

  # Renders "alert('hello')" and sets the mime type to text/javascript
  render :js => "alert('hello')"

Rendering with status and location headers

All renders take the :status and :location options and turn them into headers. They can even be used together:

  render :xml => post.to_xml, :status => :created, :location => post_url(post)
     # File actionpack/lib/action_controller/base.rb, line 890
890:       def render(options = nil, extra_options = {}, &block) #:doc:
891:         raise DoubleRenderError, "Can only render or redirect once per action" if performed?
893:         validate_render_arguments(options, extra_options, block_given?)
895:         if options.nil?
896:           options = { :template => default_template, :layout => true }
897:         elsif options == :update
898:           options = extra_options.merge({ :update => true })
899:         elsif options.is_a?(String) || options.is_a?(Symbol)
900:           case options.to_s.index('/')
901:           when 0
902:             extra_options[:file] = options
903:           when nil
904:             extra_options[:action] = options
905:           else
906:             extra_options[:template] = options
907:           end
909:           options = extra_options
910:         elsif !options.is_a?(Hash)
911:           extra_options[:partial] = options
912:           options = extra_options
913:         end
915:         layout = pick_layout(options)
916:         response.layout = layout.path_without_format_and_extension if layout
917:"Rendering template within #{layout.path_without_format_and_extension}") if logger && layout
919:         if content_type = options[:content_type]
920:           response.content_type = content_type.to_s
921:         end
923:         if location = options[:location]
924:           response.headers["Location"] = url_for(location)
925:         end
927:         if options.has_key?(:text)
928:           text = layout ? @template.render(options.merge(:text => options[:text], :layout => layout)) : options[:text]
929:           render_for_text(text, options[:status])
931:         else
932:           if file = options[:file]
933:             render_for_file(file, options[:status], layout, options[:locals] || {})
935:           elsif template = options[:template]
936:             render_for_file(template, options[:status], layout, options[:locals] || {})
938:           elsif inline = options[:inline]
939:             render_for_text(@template.render(options.merge(:layout => layout)), options[:status])
941:           elsif action_name = options[:action]
942:             render_for_file(default_template(action_name.to_s), options[:status], layout)
944:           elsif xml = options[:xml]
945:             response.content_type ||= Mime::XML
946:             render_for_text(xml.respond_to?(:to_xml) ? xml.to_xml : xml, options[:status])
948:           elsif js = options[:js]
949:             response.content_type ||= Mime::JS
950:             render_for_text(js, options[:status])
952:           elsif options.include?(:json)
953:             json = options[:json]
954:             json = ActiveSupport::JSON.encode(json) unless json.is_a?(String)
955:             json = "#{options[:callback]}(#{json})" unless options[:callback].blank?
956:             response.content_type ||= Mime::JSON
957:             render_for_text(json, options[:status])
959:           elsif options[:partial]
960:             options[:partial] = default_template_name if options[:partial] == true
961:             if layout
962:               render_for_text(@template.render(:text => @template.render(options), :layout => layout), options[:status])
963:             else
964:               render_for_text(@template.render(options), options[:status])
965:             end
967:           elsif options[:update]
968:             @template.send(:_evaluate_assigns_and_ivars)
970:             generator =, &block)
971:             response.content_type = Mime::JS
972:             render_for_text(generator.to_s, options[:status])
974:           elsif options[:nothing]
975:             render_for_text(nil, options[:status])
977:           else
978:             render_for_file(default_template, options[:status], layout)
979:           end
980:         end
981:       end
render_to_string(options = nil, &block)

Renders according to the same rules as render, but returns the result in a string instead of sending it as the response body to the browser.

     # File actionpack/lib/action_controller/base.rb, line 985
985:       def render_to_string(options = nil, &block) #:doc:
986:         render(options, &block)
987:       ensure
988:         response.content_type = nil
989:         erase_render_results
990:         reset_variables_added_to_assigns
991:       end

Resets the session by clearing out all the objects stored within and initializing a new session object.

      # File actionpack/lib/action_controller/base.rb, line 1241
1241:       def reset_session #:doc:
1242:         request.reset_session
1243:         @_session = request.session
1244:       end

Sets the etag and/or last_modified on the response and checks it against the client request. If the request doesn‘t match the options provided, the request is considered stale and should be generated from scratch. Otherwise, it‘s fresh and we don‘t need to generate anything and a reply of "304 Not Modified" is sent.


  • :etag
  • :last_modified
  • :public By default the Cache-Control header is private, set this to true if you want your application to be cachable by other devices (proxy caches).


  def show
    @article = Article.find(params[:id])

    if stale?(:etag => @article, :last_modified => @article.created_at.utc)
      @statistics = @article.really_expensive_call
      respond_to do |format|
        # all the supported formats
      # File actionpack/lib/action_controller/base.rb, line 1164
1164:       def stale?(options)
1165:         fresh_when(options)
1166:         !request.fresh?(response)
1167:       end