What does a controller do?

Action Controller is the C in MVC. After routing has determined which controller to use for a request, your controller is responsible for making sense of the request and producing the appropriate output. Luckily, Action Controller does most of the groundwork for you and uses smart conventions to make this as straight-forward as possible.

For most conventional RESTful applications, the controller will receive the request (this is invisible to you as the developer), fetch or save data from a model and use a view to create HTML output. If your controller needs to do things a little differently, that's not a problem, this is just the most common way for a controller to work.

A controller can thus be thought of as a middle man between models and views. It makes the model data available to the view so it can display it to the user, and it saves or updates data from the user to the model.

Methods and actions

A controller is a Ruby class which inherits from ActionController::Base and has methods just like any other class. Usually these methods correspond to actions in MVC, but they can just as well be helpful methods which can be called by actions. When your application receives a request, the routing will determine which controller and action to run. Then an i nstance of that controller will be created and the method corresponding to the action (the method with the same name as the action) gets run.

class ClientsController < ActionController::Base

  # Actions are public methods
  def new
  end

  # These methods are responsible for producing output
  def edit
  end

# Helper methods are private and can not be used as actions
private

  def foo
  end

end

Private methods in a controller are also used as filters, which will be covered later in this guide.

As an example, if the user goes to `/clients/new` in your application to add a new client, a ClientsController instance will be created and the `new` method will be run. Note that the empty method from the example above could work just fine because Rails will by default render the `new.html.erb` view unless the action says otherwise. The `new` method could make available to the view a `@client` instance variable by creating a new Client:

def new
  @client = Client.new
end

The Layouts & rendering guide explains this in more detail.

Articles in this guide

1. Introduction
2. What does a controller do? (This article)
3. Parameters
4. Sessions
5. Cookies
6. Filters
7. Verification
8. The request and response objects
9. HTTP Basic Authentication
10. Streaming and file downloads
11. Parameter filtering
12. Rescue

Thanks to the Ruby on Rails documentation team

This guide was written by Tore Darrell as part of the Ruby on Rails Documentation Project and is provided freely under a Creative Commons licence