Action Pack

We have already considered how models (the 'M' in MVC) are supported by the Ruby ActiveRecord module in Rails.

Views and controllers in Rails are supported by Action Pack, which consists of three Ruby modules: ActionDispatch, ActionController and ActionView.

Controllers are the heart of a Rails application – when a user connects to your Rails application, they do it by asking the application to execute a controller action.

How does Rails determine the controller action it will execute?


When an HTTP request is made to a Rails application, the ActionDispatch module is used to map that request to a particular controller action.

Requests are mapped to controller actions via the routes defined in ./config/routes.rb file. We already seen that you can view the routes defined in your application by executing:

$ rake routes

To connect a request to a controller action, you add a route to ./config/routes.rb. Examples of the various ways you can do this are provided as comments in that file.


Ex. Adding the following to ./config/routes.rb:

get ’products/:id’ => ’catalog#view’

will map a GET request that uses the URL:

to the view method in the CatalogContoller class, assigning the value 10 to the params[:id] hash that will be made available to all methods in that class.

This doesn’t look like the syntax used by the scaffold generator to create the routes for our posts and comments:

resources :posts
resources :comments

RESTful Controllers

By default, Rails controllers are RESTful and therefore use resource routing.

REST stands for Representational State Transfer.

The fundamental philosophy behind REST is that clients should communicate with servers through stateless connections, where:

  • – Long term state is kept on the server side by maintaining a set of identifiable resources, posts and comments in our case.
  • – The client can access these resources (perform CRUD operations on them) through a highly limited but uniform interface (a set of URLs in our case).
  • – Computation proceeds by identifying the resource and the CRUD operation you’d like to perform on it.

A REST-based web application can be contrasted to a RPC-based web application.

  • – In RPC-based applications, clients send requests to servers, asking them to execute a specified procedure (available on the server) using the supplied parameters. The server must advertise the services it offers. SOAP is a protocol, developed by Microsoft, that supports this approach.
  • – REST assumes a simple set of verbs (controller actions/methods) that can operate over a rich set of nouns (resources).
  • – RPC allows for arbitrary complexity on the server side.

The constraints imposed by REST can lead to web applications that easier-to-write and maintain. Rather than implementing remotely accessible services, a simple interface for performing CRUD operations on resources is provided.

The Programmable Web, which treats the WWW as a vast collection of addressable resources, is greatly facilitated by REST.

Resourceful Routes

The statement in ./config/routes.rb:

resources :posts
produces seven different routes in your application, all mapping to methods in the PostsController class:

HTTP VerbPathMethodPurpose
GET/postsindexdispaly all posts
GET/posts/new newreturn form for creating a post
POST/postscreatecreate a new post
GET/posts/:idshowdisplay a specific post
GET/posts/:id/editeditreturn form for editing a post
PATCH/PUT/posts/:idupdateupdate a specific post
DELETE/posts/:iddestroydelete a specific post