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.

By default, only the ApplicationController in a Rails application inherits from ActionController::Base. All other controllers in turn inherit from ApplicationController. This gives you one class to configure things such as request forgery protection and filtering of sensitive request parameters.

A sample controller could look like this:

class PostsController < ApplicationController
  def index
    @posts = Post.all

  def create
    @post = Post.create params[:post]
    redirect_to posts_path

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 PostsController would render the template app/views/posts/index.html.erb by default after populating the @posts instance variable.

Unlike index, the create action will not render a template. After performing its main purpose (creating a new post), it initiates a redirect instead. This redirect works by returning an external “302 Moved” HTTP response that takes the user to the index action.

These two methods represent the two basic action archetypes used in Action Controllers. Get-and-show and do-and-redirect. Most actions are variations on these themes.


For every request, the router determines the value of the controller and action keys. These determine which controller and action are called. 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 accessor methods. Then the action is performed.

The full request object is available via the request accessor and is primarily used to query for HTTP headers:

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 /posts?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 allow 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:

  • 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

    MyApplication::Application.config.session_store :active_record_store

    in your config/initializers/session_store.rb and run script/rails g session_migration.


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. For example, 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.count
    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 ActionView::Base.


Redirects are used to move from one action to another. For example, after a create action, which stores a blog entry to the 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. Note that this is an external HTTP-level redirection which will cause the browser to make a second request (a GET to the show action), and not some internal re-routing which calls both “create” and then “show” within one request.

Learn more about redirect_to and what options you have in ActionController::Redirecting.

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


MODULES = [ AbstractController::Layouts, AbstractController::Translation, AbstractController::AssetPaths, Helpers, HideActions, UrlFor, Redirecting, Rendering, Renderers::All, ConditionalGet, RackDelegation, Caching, MimeResponds, ImplicitRender, Cookies, Flash, RequestForgeryProtection, ForceSSL, Streaming, DataStreaming, RecordIdentifier, HttpAuthentication::Basic::ControllerMethods, HttpAuthentication::Digest::ControllerMethods, HttpAuthentication::Token::ControllerMethods, # Before callbacks should also be executed the earliest as possible, so # also include them at the bottom. AbstractController::Callbacks, # Append rescue at the bottom to wrap as much as possible. Rescue, # Add instrumentations hooks at the bottom, to ensure they instrument # all the methods properly. Instrumentation, # Params wrapper should come before instrumentation so they are # properly showed in logs ParamsWrapper ]


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July 23, 2008
4 thanks

Keep your controllers clear

When you use redirect_to or render with flash[:notice] or flash[:error], you can define some helper methods in your ApplicationController (or somewhere you want):

class ApplicationController < ActionController::Base


    %w(notice error).each do |message|
      class_eval <<-END_EVAL
        def redirect_#{message}(url, message)
          flash[:#{message}] = message
          redirect_to url

        def render_#{message}(action, message)
          flash[:#{message}] = message
          render :action => action

Now you have four methods - redirect_notice, redirect_error, render_notice and render_error.

November 7, 2008
2 thanks

Parsing YAML from a POST request

When building a REST server which should accept YAML there are several things to take into consideration.

First of the client should tell the server what type of data it is going to send. This is done via the Content-Type header (which is NOT only a response header as opposed to what the RESTful Web Services book from O’Reilly made us believe).

Second the server application should know how handle the body of the POST request. Placing the following line in your environment.rb:

ActionController::Base.param_parsers[Mime::YAML] = :yaml

This registers the YAML parser. Smooth sailing from here on!

June 1, 2009
2 thanks

Make sure your action names don't step on any toes.

In my experience, if you ever have a controller action named “process”, your controller will cease to function, as there is both a class and instance method called process in ActionController::Base.

There are undoubtedly other action names that will cause conflicts, but this one is particular I’ve run into a number of times.

September 1, 2009
1 thank

session expiration

If you need to set expiration period for sessions through all controllers in your application, simply add the following option to your config/intializers/session_store.rb file:

:expire_after => 60.minutes

If you need to set different expiration time in different controllers or actions, use the following code in action or some before_filter:

request.session_options = request.session_options.dup
request.session_options[:expire_after] = 5.minutes

Duplication of the hash is needed only because it is already frozen at that point, even though modification of at least :expire_after is possible and works flawlessly.

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