Action Pack – On rails from request to response

Action Pack splits the response to a web request into a controller part (performing the logic) and a view part (rendering a template). This two-step approach is known as an action, which will normally create, read, update, or delete (CRUD for short) some sort of model part (often backed by a database) before choosing either to render a template or redirecting to another action.

Action Pack implements these actions as public methods on Action Controllers and uses Action Views to implement the template rendering. Action Controllers are then responsible for handling all the actions relating to a certain part of an application. This grouping usually consists of actions for lists and for CRUDs revolving around a single (or a few) model objects. So ContactController would be responsible for listing contacts, creating, deleting, and updating contacts. A WeblogController could be responsible for both posts and comments.

Action View templates are written using embedded Ruby in tags mingled in with the HTML. To avoid cluttering the templates with code, a bunch of helper classes provide common behavior for forms, dates, and strings. And it’s easy to add specific helpers to keep the separation as the application evolves.

Note: Some of the features, such as scaffolding and form building, are tied to ActiveRecord (an object-relational mapping package), but that doesn’t mean that Action Pack depends on Active Record. Action Pack is an independent package that can be used with any sort of backend (Instiki, which is based on an older version of Action Pack, used Madeleine for example). Read more about the role Action Pack can play when used together with Active Record on www.rubyonrails.org.

A short rundown of the major features:

Actions grouped in controller as methods instead of separate command objects and can therefore share helper methods.

BlogController < ActionController::Base
  def show
    @customer = find_customer
  end

  def update
    @customer = find_customer
    @customer.attributes = params[:customer]
    @customer.save ? 
      redirect_to(:action => "display") : 
      render(:action => "edit")
  end

  private
    def find_customer() Customer.find(params[:id]) end
end

Embedded Ruby for templates (no new “easy” template language)

<% for post in @posts %>
  Title: <%= post.title %>
<% end %>

All post titles: <%= @post.collect{ |p| p.title }.join ", " %>

<% unless @person.is_client? %>
  Not for clients to see...
<% end %>

Builder-based templates (great for XML content, like RSS)

xml.rss("version" => "2.0") do
  xml.channel do
    xml.title(@feed_title)
    xml.link(@url)
    xml.description "Basecamp: Recent items"
    xml.language "en-us"
    xml.ttl "40"

    for item in @recent_items
      xml.item do
        xml.title(item_title(item))
        xml.description(item_description(item))
        xml.pubDate(item_pubDate(item))
        xml.guid(@recent_items.url(item))
        xml.link(@recent_items.url(item))
      end
    end
  end
end

Filters for pre and post processing of the response (as methods, procs, and classes)

class WeblogController < ActionController::Base
  before_filter :authenticate, :cache, :audit
  after_filter { |c| c.response.body = Gzip::compress(c.response.body) }
  after_filter LocalizeFilter

  def index
    # Before this action is run, the user will be authenticated, the cache
    # will be examined to see if a valid copy of the results already
    # exists, and the action will be logged for auditing.

    # After this action has run, the output will first be localized then 
    # compressed to minimize bandwidth usage
  end

  private
    def authenticate
      # Implement the filter with full access to both request and response
    end
end

Helpers for forms, dates, action links, and text

<%= text_field "post", "title", "size" => 30 %>
<%= html_date_select(Date.today) %>
<%= link_to "New post", :controller => "post", :action => "new" %>
<%= truncate(post.title, 25) %>

Layout sharing for template reuse (think simple version of Struts Tiles)

class WeblogController < ActionController::Base
  layout "weblog_layout"

  def hello_world
  end
end

Layout file (called weblog_layout):
  <html><body><%= yield %></body></html>

Template for hello_world action:
  <h1>Hello world</h1>

Result of running hello_world action:
  <html><body><h1>Hello world</h1></body></html>

Routing makes pretty urls incredibly easy

map.connect 'clients/:client_name/:project_name/:controller/:action'

Accessing /clients/37signals/basecamp/project/dash calls ProjectController#dash with
{ "client_name" => "37signals", "project_name" => "basecamp" } in params[:params]

From that URL, you can rewrite the redirect in a number of ways:

redirect_to(:action => "edit") =>
  /clients/37signals/basecamp/project/dash

redirect_to(:client_name => "nextangle", :project_name => "rails") =>
  /clients/nextangle/rails/project/dash

Javascript and Ajax integration.

link_to_function "Greeting", "alert('Hello world!')"
link_to_remote "Delete this post", :update => "posts", 
               :url => { :action => "destroy", :id => post.id }

Pagination for navigating lists of results.

# controller
def list
  @pages, @people =
    paginate :people, :order => 'last_name, first_name'
end

# view
<%= link_to "Previous page", { :page => @pages.current.previous } if @pages.current.previous %>
<%= link_to "Next page", { :page => @pages.current.next } if @pages.current.next %>

Easy testing of both controller and template result through TestRequest/Response

class LoginControllerTest < Test::Unit::TestCase
  def setup
    @controller = LoginController.new
    @request    = ActionController::TestRequest.new
    @response   = ActionController::TestResponse.new
  end

  def test_failing_authenticate
    process :authenticate, :user_name => "nop", :password => ""
    assert flash.has_key?(:alert)
    assert_redirected_to :action => "index"
  end
end

Automated benchmarking and integrated logging

Processing WeblogController#index (for 127.0.0.1 at Fri May 28 00:41:55)
Parameters: {"action"=>"index", "controller"=>"weblog"}
Rendering weblog/index (200 OK)
Completed in 0.029281 (34 reqs/sec)

If Active Record is used as the model, you'll have the database debugging
as well:

Processing WeblogController#create (for 127.0.0.1 at Sat Jun 19 14:04:23)
Params: {"controller"=>"weblog", "action"=>"create",  
         "post"=>{"title"=>"this is good"} }
SQL (0.000627) INSERT INTO posts (title) VALUES('this is good')
Redirected to http://test/weblog/display/5
Completed in 0.221764 (4 reqs/sec) | DB: 0.059920 (27%)

You specify a logger through a class method, such as:

ActionController::Base.logger = Logger.new("Application Log")
ActionController::Base.logger = Log4r::Logger.new("Application Log")

Caching at three levels of granularity (page, action, fragment)

class WeblogController < ActionController::Base
  caches_page :show
  caches_action :account

  def show
    # the output of the method will be cached as 
    # ActionController::Base.page_cache_directory + "/weblog/show/n.html"
    # and the web server will pick it up without even hitting Rails
  end

  def account
    # the output of the method will be cached in the fragment store
    # but Rails is hit to retrieve it, so filters are run
  end

  def update
    List.update(params[:list][:id], params[:list])
    expire_page   :action => "show", :id => params[:list][:id]
    expire_action :action => "account"
    redirect_to   :action => "show", :id => params[:list][:id]
  end
end

Component requests from one controller to another

class WeblogController < ActionController::Base
  # Performs a method and then lets hello_world output its render
  def delegate_action
    do_other_stuff_before_hello_world
    render_component :controller => "greeter",  :action => "hello_world"
  end
end

class GreeterController < ActionController::Base
  def hello_world
    render_text "Hello World!"
  end
end

The same can be done in a view to do a partial rendering:

  Let's see a greeting:
  <%= render_component :controller => "greeter", :action => "hello_world" %>

Powerful debugging mechanism for local requests

All exceptions raised on actions performed on the request of a local user
will be presented with a tailored debugging screen that includes exception
message, stack trace, request parameters, session contents, and the
half-finished response.

Scaffolding for Active Record model objects

class AccountController < ActionController::Base
  scaffold :account
end

The AccountController now has the full CRUD range of actions and default
templates: list, show, destroy, new, create, edit, update

Form building for Active Record model objects

The post object has a title (varchar), content (text), and 
written_on (date)

<%= form "post" %>

...will generate something like (the selects will have more options, of
course):

<form action="create" method="POST">
  <p>
    <b>Title:</b><br/> 
    <input type="text" name="post[title]" value="<%= @post.title %>" />
  </p>
  <p>
    <b>Content:</b><br/>
    <textarea name="post[content]"><%= @post.title %></textarea>
  </p>
  <p>
    <b>Written on:</b><br/>
    <select name='post[written_on(3i)]'><option>18</option></select>
    <select name='post[written_on(2i)]'><option value='7'>July</option></select>
    <select name='post[written_on(1i)]'><option>2004</option></select>
  </p>

  <input type="submit" value="Create">
</form>

This form generates a params[:post] array that can be used directly in a save action:

class WeblogController < ActionController::Base
  def create
    post = Post.create(params[:post])
    redirect_to :action => "display", :id => post.id
  end
end

Runs on top of WEBrick, Mongrel, CGI, FCGI, and mod_ruby

Simple example (from outside of Rails)

This example will implement a simple weblog system using inline templates and an Active Record model. So let’s build that WeblogController with just a few methods:

require 'action_controller'
require 'post'

class WeblogController < ActionController::Base
  layout "weblog/layout"

  def index
    @posts = Post.find(:all)
  end

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

  def new
    @post = Post.new
  end

  def create
    @post = Post.create(params[:post])
    redirect_to :action => "display", :id => @post.id
  end
end

WeblogController::Base.view_paths = [ File.dirname(__FILE__) ]
WeblogController.process_cgi if $0 == __FILE__

The last two lines are responsible for telling ActionController where the template files are located and actually running the controller on a new request from the web-server (like to be Apache).

And the templates look like this:

weblog/layout.erb:
  <html><body>
  <%= yield %>
  </body></html>

weblog/index.erb:
  <% for post in @posts %>
    <p><%= link_to(post.title, :action => "display", :id => post.id %></p>
  <% end %>

weblog/display.erb:
  <p>
    <b><%= post.title %></b><br/>
    <b><%= post.content %></b>
  </p>

weblog/new.erb:
  <%= form "post" %>

This simple setup will list all the posts in the system on the index page, which is called by accessing /weblog/. It uses the form builder for the Active Record model to make the new screen, which in turn hands everything over to the create action (that’s the default target for the form builder when given a new model). After creating the post, it’ll redirect to the display page using an URL such as /weblog/display/5 (where 5 is the id of the post).

Examples

Action Pack ships with three examples that all demonstrate an increasingly detailed view of the possibilities. First is blog_controller that is just a single file for the whole MVC (but still split into separate parts). Second is the debate_controller that uses separate template files and multiple screens. Third is the address_book_controller that uses the layout feature to separate template casing from content.

Please note that you might need to change the “shebang” line to #!/usr/local/env ruby, if your Ruby is not placed in /usr/local/bin/ruby

Also note that these examples are all for demonstrating using Action Pack on its own. Not for when it’s used inside of Rails.

Download

The latest version of Action Pack can be found at

rubyforge.org/project/showfiles.php?group_id=249

Documentation can be found at

api.rubyonrails.com

Installation

You can install Action Pack with the following command.

% [sudo] ruby install.rb

from its distribution directory.

License

Action Pack is released under the MIT license.

Support

The Action Pack homepage is www.rubyonrails.org. You can find the Action Pack RubyForge page at rubyforge.org/projects/actionpack. And as Jim from Rake says:

Feel free to submit commits or feature requests.  If you send a patch,
remember to update the corresponding unit tests.  If fact, I prefer
new feature to be submitted in the form of new unit tests.