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 it renders a template or redirects 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 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
  end

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

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.

Requests

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["REMOTE_ADDR"]
  render plain: "This server hosted at #{location}"
end

Parameters

All request parameters, whether they come from a query string in the URL or form data submitted through a POST request 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 coming 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

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)

You can retrieve it 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 them to see its contents.

Do not put secret information in cookie-based sessions!

Responses

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.

Renders

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])
end

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"
  end
end

Read more about writing ERB and Builder templates in ActionView::Base.

Redirects

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.new(params[:entry])
  if @entry.save
    # The entry was saved correctly, redirect to show
    redirect_to action: 'show', id: @entry.id
  else
    # things didn't go so well, do something else
  end
end

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
end

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
end