<commit_before>h2. Getting Started with Rails

This guide covers getting up and running with Ruby on Rails. After reading it,
you should be familiar with:

* Installing Rails, creating a new Rails application, and connecting your application to a database
* The general layout of a Rails application
* The basic principles of MVC (Model, View Controller) and RESTful design
* How to quickly generate the starting pieces of a Rails application

endprologue.

WARNING. This Guide is based on Rails 3.1. Some of the code shown here will not
work in earlier versions of Rails.

h3. Guide Assumptions

This guide is designed for beginners who want to get started with a Rails
application from scratch. It does not assume that you have any prior experience
with Rails. However, to get the most out of it, you need to have some
prerequisites installed:

* The "Ruby":http://www.ruby-lang.org/en/downloads language version 1.8.7 or higher

TIP: Note that Ruby 1.8.7 p248 and p249 have marshaling bugs that crash Rails
3.0. Ruby Enterprise Edition have these fixed since release 1.8.7-2010.02
though. On the 1.9 front, Ruby 1.9.1 is not usable because it outright segfaults
on Rails 3.0, so if you want to use Rails 3 with 1.9.x jump on 1.9.2 for smooth
sailing.

* The "RubyGems":http://rubyforge.org/frs/?group_id=126 packaging system
  ** If you want to learn more about RubyGems, please read the "RubyGems User Guide":http://docs.rubygems.org/read/book/1
* A working installation of the "SQLite3 Database":http://www.sqlite.org

Rails is a web application framework running on the Ruby programming language.
If you have no prior experience with Ruby, you will find a very steep learning
curve diving straight into Rails. There are some good free resources on the
internet for learning Ruby, including:

* "Mr. Neighborly's Humble Little Ruby Book":http://www.humblelittlerubybook.com
* "Programming Ruby":http://www.ruby-doc.org/docs/ProgrammingRuby/
* "Why's (Poignant) Guide to Ruby":http://mislav.uniqpath.com/poignant-guide/

h3. What is Rails?

Rails is a web application development framework written in the Ruby language.
It is designed to make programming web applications easier by making assumptions
about what every developer needs to get started. It allows you to write less
code while accomplishing more than many other languages and frameworks.
Experienced Rails developers also report that it makes web application
development more fun.

Rails is opinionated software. It makes the assumption that there is a "best"
way to do things, and it's designed to encourage that way - and in some cases to
discourage alternatives. If you learn "The Rails Way" you'll probably discover a
tremendous increase in productivity. If you persist in bringing old habits from
other languages to your Rails development, and trying to use patterns you
learned elsewhere, you may have a less happy experience.

The Rails philosophy includes several guiding principles:

* DRY - "Don't Repeat Yourself" - suggests that writing the same code over and over again is a bad thing.
* Convention Over Configuration - means that Rails makes assumptions about what you want to do and how you're going to
do it, rather than requiring you to specify every little thing through endless configuration files.
* REST is the best pattern for web applications - organizing your application around resources and standard HTTP verbs
is the fastest way to go.

h4. The MVC Architecture

At the core of Rails is the Model, View, Controller architecture, usually just
called MVC. MVC benefits include:

* Isolation of business logic from the user interface
* Ease of keeping code DRY
* Making it clear where different types of code belong for easier maintenance

h5. Models

A model represents the information (data) of the application and the rules to
manipulate that data. In the case of Rails, models are primarily used for
managing the rules of interaction with a corresponding database table. In most
cases, each table in your database will correspond to one model in your
application. The bulk of your application's business logic will be concentrated
in the models.

h5. Views

Views represent the user interface of your application. In Rails, views are
often HTML files with embedded Ruby code that perform tasks related solely to
the presentation of the data. Views handle the job of providing data to the web
browser or other tool that is used to make requests from your application.

h5. Controllers

Controllers provide the "glue" between models and views. In Rails, controllers
are responsible for processing the incoming requests from the web browser,
interrogating the models for data, and passing that data on to the views for
presentation.

h4. The Components of Rails

Rails ships as many individual components.  Each of these components are briefly
explained below.  If you are new to Rails, as you read this section, don't get
hung up on the details of each component, as they will be explained in further
detail later.  For instance, we will bring up Rack applications, but you don't
need to know anything about them to continue with this guide.

* Action Pack
  ** Action Controller
  ** Action Dispatch
  ** Action View
* Action Mailer
* Active Model
* Active Record
* Active Resource
* Active Support
* Railties

h5. Action Pack

Action Pack is a single gem that contains Action Controller, Action View and
Action Dispatch. The "VC" part of "MVC".

h6. Action Controller

Action Controller is the component that manages the controllers in a Rails
application. The Action Controller framework processes incoming requests to a
Rails application, extracts parameters, and dispatches them to the intended
action.  Services provided by Action Controller include session management,
template rendering, and redirect management.

h6. Action View

Action View manages the views of your Rails application. It can create both HTML
and XML output by default. Action View manages rendering templates, including
nested and partial templates, and includes built-in AJAX support.  View
templates are covered in more detail in another guide called "Layouts and
Rendering":layouts_and_rendering.html.

h6. Action Dispatch

Action Dispatch handles routing of web requests and dispatches them as you want,
either to your application or any other Rack application.  Rack applications are
a more advanced topic and are covered in a separate guide called "Rails on
Rack":rails_on_rack.html.

h5. Action Mailer

Action Mailer is a framework for building e-mail services. You can use Action
Mailer to receive and process incoming email and send simple plain text or
complex multipart emails based on flexible templates.

h5. Active Model

Active Model provides a defined interface between the Action Pack gem services
and Object Relationship Mapping gems such as Active Record. Active Model allows
Rails to utilize other ORM frameworks in place of Active Record if your
application needs this.

h5. Active Record

Active Record is the base for the models in a Rails application. It provides
database independence, basic CRUD functionality, advanced finding capabilities,
and the ability to relate models to one another, among other services.

h5. Active Resource

Active Resource provides a framework for managing the connection between
business objects and RESTful web services. It implements a way to map web-based
resources to local objects with CRUD semantics.

h5. Active Support

Active Support is an extensive collection of utility classes and standard Ruby
library extensions that are used in Rails, both by the core code and by your
applications.

h5. Railties

Railties is the core Rails code that builds new Rails applications and glues the
various frameworks and plugins together in any Rails application.

h4. REST

Rest stands for Representational State Transfer and is the foundation of the
RESTful architecture. This is generally considered to be Roy Fielding's doctoral
thesis, "Architectural Styles and the Design of Network-based Software
Architectures":http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm. While
you can read through the thesis, REST in terms of Rails boils down to two main
principles:

* Using resource identifiers such as URLs to represent resources.
* Transferring representations of the state of that resource between system components.

For example, the following HTTP request:

<tt>DELETE /photos/17</tt>

would be understood to refer to a photo resource with the ID of 17, and to
indicate a desired action - deleting that resource. REST is a natural style for
the architecture of web applications, and Rails hooks into this shielding you
from many of the RESTful complexities and browser quirks.

If you'd like more details on REST as an architectural style, these resources
are more approachable than Fielding's thesis:

* "A Brief Introduction to REST":http://www.infoq.com/articles/rest-introduction by <NAME>
* "An Introduction to REST":http://bitworking.org/news/373/An-Introduction-to-REST (video tutorial) by <NAME>
* "Representational State Transfer":http://en.wikipedia.org/wiki/Representational_State_Transfer article in Wikipedia
* "How to GET a Cup of Coffee":http://www.infoq.com/articles/webber-rest-workflow by <NAME>, <NAME> &
<NAME>

h3. Creating a New Rails Project

If you follow this guide, you'll create a Rails project called <tt>blog</tt>, a
(very) simple weblog. Before you can start building the application, you need to
make sure that you have Rails itself installed.

TIP: The examples below use # and $ to denote terminal prompts. If you are using Windows, your prompt will look something like c:\source_code>

h4. Installing Rails

In most cases, the easiest way to install Rails is to take advantage of RubyGems:

<shell>
Usually run this as the root user:
# gem install rails
</shell>

TIP. If you're working on Windows, you can quickly install Ruby and Rails with
"Rails Installer":http://railsinstaller.org.

h4. Creating the Blog Application

The best way to use this guide is to follow each step as it happens, no code or
step needed to make this example application has been left out, so you can
literally follow along step by step. If you need to see the completed code, you
can download it from "Getting Started
Code":https://github.com/mikel/getting-started-code.

To begin, open a terminal, navigate to a folder where you have rights to create
files, and type:

<shell>
$ rails new blog
</shell>

This will create a Rails application called Blog in a directory called blog.

TIP: You can see all of the switches that the Rails application builder accepts
by running
<tt>rails new -h</tt>.

After you create the blog application, switch to its folder to continue work
directly in that application:

<shell>
$ cd blog
</shell>

In any case, Rails will create a folder in your working directory called
<tt>blog</tt>. Open up that folder and explore its contents. Most of the work in
this tutorial will happen in the <tt>app/</tt> folder, but here's a basic
rundown on the function of each folder that Rails creates in a new application
by default:

|_.File/Folder|_.Purpose|
|Gemfile|This file allows you to specify what gem dependencies are needed for your Rails application. See section on Bundler, below.|
|README|This is a brief instruction manual for your application. You should edit this file to tell others what your application does, how to set it up, and so on.|
|Rakefile|This file locates and loads tasks that can be run from the command line. The task definitions are defined throughout the components of Rails. Rather than changing Rakefile, you should add your own tasks by adding files to the lib/tasks directory of your application.|
|app/|Contains the controllers, models, views and assets for your application. You'll focus on this folder for the remainder of this guide.|
|config/|Configure your application's runtime rules, routes, database, and more.|
|config.ru|Rack configuration for Rack based servers used to start the application.|
|db/|Shows your current database schema, as well as the database migrations. You'll learn about migrations shortly.|
|doc/|In-depth documentation for your application.|
|lib/|Extended modules for your application (not covered in this guide).|
|log/|Application log files.|
|public/|The only folder seen to the world as-is. Contains the static files and compiled assets.|
|script/|Contains the rails script that starts your app and can contain other scripts you use to deploy or run your application.|
|test/|Unit tests, fixtures, and other test apparatus. These are covered in "Testing Rails Applications":testing.html|
|tmp/|Temporary files|
|vendor/|A place for all third-party code. In a typical Rails application, this includes Ruby Gems, the Rails source code (if you install it into your project) and plugins containing additional prepackaged functionality.|

h4. Configuring a Database

Just about every Rails application will interact with a database. The database
to use is specified in a configuration file, +config/database.yml+.  If you open
this file in a new Rails application, you'll see a default database
configuration using SQLite3. The file contains sections for three different
environments in which Rails can run by default:

* The +development+ environment is used on your development computer as you interact manually with the application.
* The +test+ environment is used to run automated tests.
* The +production+ environment is used when you deploy your application for the world to use.

h5. Configuring an SQLite3 Database

Rails comes with built-in support for "SQLite3":http://www.sqlite.org, which is
a lightweight serverless database application. While a busy production
environment may overload SQLite, it works well for development and testing.
Rails defaults to using an SQLite database when creating a new project, but you
can always change it later.

Here's the section of the default configuration file
(<tt>config/database.yml</tt>) with connection information for the development
environment:

<yaml>
development:
  adapter: sqlite3
  database: db/development.sqlite3
  pool: 5
  timeout: 5000
</yaml>

NOTE: In this guide we are using an SQLite3 database for data storage, because
it is a zero configuration database that just works. Rails also supports MySQL
and PostgreSQL "out of the box", and has plugins for many database systems. If
you are using a database in a production environment Rails most likely has an
adapter for it.

h5. Configuring a MySQL Database

If you choose to use MySQL instead of the shipped SQLite3 database, your
+config/database.yml+ will look a little different. Here's the development
section:

<yaml>
development:
  adapter: mysql2
  encoding: utf8
  database: blog_development
  pool: 5
  username: root
  password:
  socket: /tmp/mysql.sock
</yaml>

If your development computer's MySQL installation includes a root user with an
empty password, this configuration should work for you. Otherwise, change the
username and password in the +development+ section as appropriate.

h5. Configuring a PostgreSQL Database

If you choose to use PostgreSQL, your +config/database.yml+ will be customized
to use PostgreSQL databases:

<yaml>
development:
  adapter: postgresql
  encoding: unicode
  database: blog_development
  pool: 5
  username: blog
  password:
</yaml>

h5. Configuring an SQLite3 Database for JRuby Platform

If you choose to use SQLite3 and using JRuby, your +config/database.yml+ will
look a little different. Here's the development section:

<yaml>
development:
  adapter: jdbcsqlite3
  database: db/development.sqlite3
</yaml>

h5. Configuring a MySQL Database for JRuby Platform

If you choose to use MySQL and using JRuby, your +config/database.yml+ will look
a little different. Here's the development section:

<yaml>
development:
  adapter: jdbcmysql
  database: blog_development
  username: root
  password:
</yaml>

h5. Configuring a PostgreSQL Database for JRuby Platform

Finally if you choose to use PostgreSQL and using JRuby, your
+config/database.yml+ will look a little different. Here's the development
section:

<yaml>
development:
  adapter: jdbcpostgresql
  encoding: unicode
  database: blog_development
  username: blog
  password:
</yaml>

Change the username and password in the +development+ section as appropriate.

TIP: You don't have to update the database configurations manually. If you look at the
options of the application generator, you will see that one of the options
is named <tt>--database</tt>. This option allows you to choose an adapter from a
list of the most used relational databases. You can even run the generator
repeatedly: <tt>cd .. && rails new blog --database=mysql</tt>. When you confirm the overwriting
 of the +config/database.yml+ file, your application will be configured for MySQL
instead of SQLite.

h4. Creating the Database

Now that you have your database configured, it's time to have Rails create an
empty database for you. You can do this by running a rake command:

<shell>
$ rake db:create
</shell>

This will create your development and test SQLite3 databases inside the
<tt>db/</tt> folder.

TIP: Rake is a general-purpose command-runner that Rails uses for many things.
You can see the list of available rake commands in your application by running
+rake -T+.

h3. Hello, Rails!

One of the traditional places to start with a new language is by getting some
text up on screen quickly. To do this, you need to get your Rails application
server running.

h4. Starting up the Web Server

You actually have a functional Rails application already. To see it, you need to
start a web server on your development machine. You can do this by running:

<shell>
$ rails server
</shell>

This will fire up an instance of the WEBrick web server by default (Rails can
also use several other web servers). To see your application in action, open a
browser window and navigate to "http://localhost:3000":http://localhost:3000.
You should see Rails' default information page:

!images/rails_welcome.png(Welcome Aboard screenshot)!

TIP: To stop the web server, hit Ctrl+C in the terminal window where it's
running. In development mode, Rails does not generally require you to stop the
server; changes you make in files will be automatically picked up by the server.

The "Welcome Aboard" page is the _smoke test_ for a new Rails application: it
makes sure that you have your software configured correctly enough to serve a
page. You can also click on the _About your application’s environment_ link to
see a summary of your application's environment.

h4. Say "Hello", Rails

To get Rails saying "Hello", you need to create at minimum a controller and a
view. Fortunately, you can do that in a single command. Enter this command in
your terminal:

<shell>
$ rails generate controller home index
</shell>

TIP: If you get a command not found error when running this command, you
need to explicitly pass Rails +rails+ commands to Ruby: <tt>ruby
\path\to\your\application\script\rails generate controller home index</tt>.

Rails will create several files for you, including
+app/views/home/index.html.erb+. This is the template that will be used to
display the results of the +index+ action (method) in the +home+ controller.
Open this file in your text editor and edit it to contain a single line of code:

<code class="html">
<h1>Hello, Rails!</h1>
</code>

h4. Setting the Application Home Page

Now that we have made the controller and view, we need to tell Rails when we
want "Hello Rails" to show up. In our case, we want it to show up when we
navigate to the root URL of our site,
"http://localhost:3000":http://localhost:3000, instead of the "Welcome Aboard"
smoke test.

The first step to doing this is to delete the default page from your
application:

<shell>
$ rm public/index.html
</shell>

We need to do this as Rails will deliver any static file in the +public+
directory in preference to any dynamic content we generate from the controllers.

Now, you have to tell Rails where your actual home page is located. Open the
file +config/routes.rb+ in your editor. This is your application's _routing
file_ which holds entries in a special DSL (domain-specific language) that tells
Rails how to connect incoming requests to controllers and actions. This file
contains many sample routes on commented lines, and one of them actually shows
you how to connect the root of your site to a specific controller and action.
Find the line beginning with +root :to+, uncomment it and change it like the
following:

<ruby>
Blog::Application.routes.draw do

  #...
  # You can have the root of your site routed with "root"
  # just remember to delete public/index.html.
  root :to => "home#index"
</ruby>

The +root :to => "home#index"+ tells Rails to map the root action to the home
controller's index action.

Now if you navigate to "http://localhost:3000":http://localhost:3000 in your
browser, you'll see +Hello, Rails!+.

NOTE. For more information about routing, refer to "Rails Routing from the
Outside In":routing.html.

h3. Getting Up and Running Quickly with Scaffolding

Rails _scaffolding_ is a quick way to generate some of the major pieces of an
application. If you want to create the models, views, and controllers for a new
resource in a single operation, scaffolding is the tool for the job.

h3. Creating a Resource

In the case of the blog application, you can start by generating a scaffolded
Post resource: this will represent a single blog posting. To do this, enter this
command in your terminal:

<shell>
$ rails generate scaffold Post name:string title:string content:text
</shell>

The scaffold generator will build several files in your application, along with some
folders, and edit <tt>config/routes.rb</tt>. Here's a quick overview of what it creates:

|_.File                                       |_.Purpose|
|db/migrate/20100207214725_create_posts.rb    |Migration to create the posts table in your database (your name will include a different timestamp)|
|app/models/post.rb                           |The Post model|
|test/fixtures/posts.yml                      |Dummy posts for use in testing|
|app/controllers/posts_controller.rb          |The Posts controller|
|app/views/posts/index.html.erb               |A view to display an index of all posts |
|app/views/posts/edit.html.erb                |A view to edit an existing post|
|app/views/posts/show.html.erb                |A view to display a single post|
|app/views/posts/new.html.erb                 |A view to create a new post|
|app/views/posts/_form.html.erb               |A partial to control the overall look and feel of the form used in edit and new views|
|app/helpers/posts_helper.rb                  |Helper functions to be used from the post views|
|app/assets/stylesheets/scaffolds.css.scss    |Cascading style sheet to make the scaffolded views look better|
|app/assets/stylesheets/posts.css.scss        |Cascading style sheet for the posts controller|
|app/assets/javascripts/posts.js.coffee       |CoffeeScript for the posts controller|
|test/unit/post_test.rb                       |Unit testing harness for the posts model|
|test/functional/posts_controller_test.rb     |Functional testing harness for the posts controller|
|test/unit/helpers/posts_helper_test.rb       |Unit testing harness for the posts helper|
|config/routes.rb                             |Edited to include routing information for posts|

NOTE. While scaffolding will get you up and running quickly, the code it
generates is unlikely to be a perfect fit for your application. You'll most
probably want to customize the generated code. Many experienced Rails developers
avoid scaffolding entirely, preferring to write all or most of their source code
from scratch. Rails, however, makes it really simple to customize templates for
generated models, controllers, views and other source files. You'll find more
information in the "Creating and Customizing Rails Generators &
Templates":generators.html guide.

h4. Running a Migration

One of the products of the +rails generate scaffold+ command is a _database
migration_. Migrations are Ruby classes that are designed to make it simple to
create and modify database tables. Rails uses rake commands to run migrations,
and it's possible to undo a migration after it's been applied to your database.
Migration filenames include a timestamp to ensure that they're processed in the
order that they were created.

If you look in the +db/migrate/20100207214725_create_posts.rb+ file (remember,
yours will have a slightly different name), here's what you'll find:

<ruby>
class CreatePosts < ActiveRecord::Migration
  def change
    create_table :posts do |t|
      t.string :name
      t.string :title
      t.text :content

      t.timestamps
    end
  end
end
</ruby>

The above migration creates a method named +change+ which will be called when you
run this migration. The action defined in that method is also reversible, which
means Rails knows how to reverse the change made by this migration, in case you
want to reverse it at later date. By default, when you run this migration it
will creates a +posts+ table with two string columns and a text column. It also
creates two timestamp fields to track record creation and updating. More
information about Rails migrations can be found in the "Rails Database
Migrations":migrations.html guide.

At this point, you can use a rake command to run the migration:

<shell>
$ rake db:migrate
</shell>

Rails will execute this migration command and tell you it created the Posts
table.

<shell>
==  CreatePosts: migrating ====================================================
-- create_table(:posts)
   -> 0.0019s
==  CreatePosts: migrated (0.0020s) ===========================================
</shell>

NOTE. Because you're working in the development environment by default, this
command will apply to the database defined in the +development+ section of your
+config/database.yml+ file. If you would like to execute migrations in other
environment, for instance in production, you must explicitly pass it when
invoking the command: <tt>rake db:migrate RAILS_ENV=production</tt>.

h4. Adding a Link

To hook the posts up to the home page you've already created, you can add a link
to the home page. Open +app/views/home/index.html.erb+ and modify it as follows:

<ruby>
<h1>Hello, Rails!</h1>
<%= link_to "My Blog", posts_path %>
</ruby>

The +link_to+ method is one of Rails' built-in view helpers. It creates a
hyperlink based on text to display and where to go - in this case, to the path
for posts.

h4. Working with Posts in the Browser

Now you're ready to start working with posts. To do that, navigate to
"http://localhost:3000":http://localhost:3000/ and then click the "My Blog"
link:

!images/posts_index.png(Posts Index screenshot)!

This is the result of Rails rendering the +index+ view of your posts. There
aren't currently any posts in the database, but if you click the +New Post+ link
you can create one. After that, you'll find that you can edit posts, look at
their details, or destroy them. All of the logic and HTML to handle this was
built by the single +rails generate scaffold+ command.

TIP: In development mode (which is what you're working in by default), Rails
reloads your application with every browser request, so there's no need to stop
and restart the web server.

Congratulations, you're riding the rails! Now it's time to see how it all works.

h4. The Model

The model file, +app/models/post.rb+ is about as simple as it can get:

<ruby>
class Post < ActiveRecord::Base
end
</ruby>

There isn't much to this file - but note that the +Post+ class inherits from
+ActiveRecord::Base+. Active Record supplies a great deal of functionality to
your Rails models for free, including basic database CRUD (Create, Read, Update,
Destroy) operations, data validation, as well as sophisticated search support
and the ability to relate multiple models to one another.

h4. Adding Some Validation

Rails includes methods to help you validate the data that you send to models.
Open the +app/models/post.rb+ file and edit it:

<ruby>
class Post < ActiveRecord::Base
  validates :name,  :presence => true
  validates :title, :presence => true,
                    :length => { :minimum => 5 }
end
</ruby>

These changes will ensure that all posts have a name and a title, and that the
title is at least five characters long. Rails can validate a variety of
conditions in a model, including the presence or uniqueness of columns, their
format, and the existence of associated objects.

h4. Using the Console

To see your validations in action, you can use the console. The console is a
command-line tool that lets you execute Ruby code in the context of your
application:

<shell>
$ rails console
</shell>

TIP: The default console will make changes to your database. You can instead
open a console that will roll back any changes you make by using +rails console
--sandbox+.

After the console loads, you can use it to work with your application's models:

<shell>
>> p = Post.new(:content => "A new post")
=> #<Post id: nil, name: nil, title: nil,
     content: "A new post", created_at: nil,
     updated_at: nil>
>> p.save
=> false
>> p.errors
=> #<OrderedHash { :title=>["can't be blank",
                           "is too short (minimum is 5 characters)"],
                   :name=>["can't be blank"] }>
</shell>

This code shows creating a new +Post+ instance, attempting to save it and
getting +false+ for a return value (indicating that the save failed), and
inspecting the +errors+ of the post.

When you're finished, type +exit+ and hit +return+ to exit the console.

TIP: Unlike the development web server, the console does not automatically load
your code afresh for each line. If you make changes to your models while the
console is open, type +reload!+ at the console prompt to load them.

h4. Listing All Posts

The easiest place to start looking at functionality is with the code that lists
all posts. Open the file +app/controllers/posts_controller.rb+ and look at the
+index+ action:

<ruby>
def index
  @posts = Post.all

  respond_to do |format|
    format.html  # index.html.erb
    format.json  { render :json => @posts }
  end
end
</ruby>

+Post.all+ calls the +Post+ model to return all of the posts currently in the
database. The result of this call is an array of posts that we store in an
instance variable called +@posts+.

TIP: For more information on finding records with Active Record, see "Active
Record Query Interface":active_record_querying.html.

The +respond_to+ block handles both HTML and JSON calls to this action. If you
browse to "http://localhost:3000/posts.json":http://localhost:3000/posts.json,
you'll see a JSON containing all of the posts. The HTML format looks for a view
in +app/views/posts/+ with a name that corresponds to the action name. Rails
makes all of the instance variables from the action available to the view.
Here's +app/views/posts/index.html.erb+:

<erb>
<h1>Listing posts</h1>

<table>
  <tr>
    <th>Name</th>
    <th>Title</th>
    <th>Content</th>
    <th></th>
    <th></th>
    <th></th>
  </tr>

<% @posts.each do |post| %>
  <tr>
    <td><%= post.name %></td>
    <td><%= post.title %></td>
    <td><%= post.content %></td>
    <td><%= link_to 'Show', post %></td>
    <td><%= link_to 'Edit', edit_post_path(post) %></td>
    <td><%= link_to 'Destroy', post, :confirm => 'Are you sure?', :method => :delete %></td>
  </tr>
<% end %>
</table>

<br />

<%= link_to 'New post', new_post_path %>
</erb>

This view iterates over the contents of the +@posts+ array to display content
and links. A few things to note in the view:

* +link_to+ builds a hyperlink to a particular destination
* +edit_post_path+ and +new_post_path+ are helpers that Rails provides as part of RESTful routing. You'll see a variety of these helpers for the different actions that the controller includes.

NOTE. In previous versions of Rails, you had to use +&lt;%=h post.name %&gt;+ so
that any HTML would be escaped before being inserted into the page. In Rails
3.0, this is now the default. To get unescaped HTML, you now use +&lt;%= raw
post.name %&gt;+.

TIP: For more details on the rendering process, see "Layouts and Rendering in
Rails":layouts_and_rendering.html.

h4. Customizing the Layout

The view is only part of the story of how HTML is displayed in your web browser.
Rails also has the concept of +layouts+, which are containers for views. When
Rails renders a view to the browser, it does so by putting the view's HTML into
a layout's HTML. In previous versions of Rails, the +rails generate scaffold+
command would automatically create a controller specific layout, like
+app/views/layouts/posts.html.erb+, for the posts controller. However this has
been changed in Rails 3.0. An application specific +layout+ is used for all the
controllers and can be found in +app/views/layouts/application.html.erb+. Open
this layout in your editor and modify the +body+ tag:

<erb>
<!DOCTYPE html>
<html>
<head>
  <title>Blog</title>
  <%= stylesheet_link_tag "application" %>
  <%= javascript_include_tag "application" %>
  <%= csrf_meta_tags %>
</head>
<body style="background: #EEEEEE;">

<%= yield %>

</body>
</html>
</erb>

Now when you refresh the +/posts+ page, you'll see a gray background to the
page. This same gray background will be used throughout all the views for posts.

h4. Creating New Posts

Creating a new post involves two actions. The first is the +new+ action, which
instantiates an empty +Post+ object:

<ruby>
def new
  @post = Post.new

  respond_to do |format|
    format.html  # new.html.erb
    format.json  { render :json => @post }
  end
end
</ruby>

The +new.html.erb+ view displays this empty Post to the user:

<erb>
<h1>New post</h1>

<%= render 'form' %>

<%= link_to 'Back', posts_path %>
</erb>

The +&lt;%= render 'form' %&gt;+ line is our first introduction to _partials_ in
Rails. A partial is a snippet of HTML and Ruby code that can be reused in
multiple locations. In this case, the form used to make a new post, is basically
identical to a form used to edit a post, both have text fields for the name and
title and a text area for the content with a button to make a new post or update
the existing post.

If you take a look at +views/posts/_form.html.erb+ file, you will see the
following:

<erb>
<%= form_for(@post) do |f| %>
  <% if @post.errors.any? %>
  <div id="errorExplanation">
    <h2><%= pluralize(@post.errors.count, "error") %> prohibited this post from being saved:</h2>
    <ul>
    <% @post.errors.full_messages.each do |msg| %>
      <li><%= msg %></li>
    <% end %>
    </ul>
  </div>
  <% end %>

  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>
  <div class="field">
    <%= f.label :title %><br />
    <%= f.text_field :title %>
  </div>
  <div class="field">
    <%= f.label :content %><br />
    <%= f.text_area :content %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>
</erb>

This partial receives all the instance variables defined in the calling view
file, so in this case, the controller assigned the new Post object to +@post+
and so, this is available in both the view and partial as +@post+.

For more information on partials, refer to the "Layouts and Rendering in
Rails":layouts_and_rendering.html#using-partials guide.

The +form_for+ block is used to create an HTML form. Within this block, you have
access to methods to build various controls on the form. For example,
+f.text_field :name+ tells Rails to create a text input on the form, and to hook
it up to the +name+ attribute of the instance being displayed. You can only use
these methods with attributes of the model that the form is based on (in this
case +name+, +title+, and +content+). Rails uses +form_for+ in preference to
having you write raw HTML because the code is more succinct, and because it
explicitly ties the form to a particular model instance.

The +form_for+ block is also smart enough to work out if you are doing a _New
Post_ or an _Edit Post_ action, and will set the form +action+ tags and submit
button names appropriately in the HTML output.

TIP: If you need to create an HTML form that displays arbitrary fields, not tied
to a model, you should use the +form_tag+ method, which provides shortcuts for
building forms that are not necessarily tied to a model instance.

When the user clicks the +Create Post+ button on this form, the browser will
send information back to the +create+ method of the controller (Rails knows to
call the +create+ method because the form is sent with an HTTP POST request;
that's one of the conventions that I mentioned earlier):

<ruby>
def create
  @post = Post.new(params[:post])

  respond_to do |format|
    if @post.save
      format.html  { redirect_to(@post,
                    :notice => 'Post was successfully created.') }
      format.json  { render :json => @post,
                    :status => :created, :location => @post }
    else
      format.html  { render :action => "new" }
      format.json  { render :json => @post.errors,
                    :status => :unprocessable_entity }
    end
  end
end
</ruby>

The +create+ action instantiates a new Post object from the data supplied by the
user on the form, which Rails makes available in the +params+ hash. After
successfully saving the new post, +create+ returns the appropriate format that
the user has requested (HTML in our case). It then redirects the user to the
resulting post +show+ action and sets a notice to the user that the Post was
successfully created.

If the post was not successfully saved, due to a validation error, then the
controller returns the user back to the +new+ action with any error messages so
that the user has the chance to fix the error and try again.

The "Post was successfully created." message is stored inside of the Rails
+flash+ hash, (usually just called _the flash_) so that messages can be carried
over to another action, providing the user with useful information on the status
of their request. In the case of +create+, the user never actually sees any page
rendered during the Post creation process, because it immediately redirects to
the new Post as soon Rails saves the record. The Flash carries over a message to
the next action, so that when the user is redirected back to the +show+ action,
they are presented with a message saying "Post was successfully created."

h4. Showing an Individual Post

When you click the +show+ link for a post on the index page, it will bring you
to a URL like +http://localhost:3000/posts/1+. Rails interprets this as a call
to the +show+ action for the resource, and passes in +1+ as the +:id+ parameter.
Here's the +show+ action:

<ruby>
def show
  @post = Post.find(params[:id])

  respond_to do |format|
    format.html  # show.html.erb
    format.json  { render :json => @post }
  end
end
</ruby>

The +show+ action uses +Post.find+ to search for a single record in the database
by its id value. After finding the record, Rails displays it by using
+show.html.erb+:

<erb>
<p class="notice"><%= notice %></p>

<p>
  <b>Name:</b>
  <%= @post.name %>
</p>

<p>
  <b>Title:</b>
  <%= @post.title %>
</p>

<p>
  <b>Content:</b>
  <%= @post.content %>
</p>


<%= link_to 'Edit', edit_post_path(@post) %> |
<%= link_to 'Back', posts_path %>
</erb>

h4. Editing Posts

Like creating a new post, editing a post is a two-part process. The first step
is a request to +edit_post_path(@post)+ with a particular post. This calls the
+edit+ action in the controller:

<ruby>
def edit
  @post = Post.find(params[:id])
end
</ruby>

After finding the requested post, Rails uses the +edit.html.erb+ view to display
it:

<erb>
<h1>Editing post</h1>

<%= render 'form' %>

<%= link_to 'Show', @post %> |
<%= link_to 'Back', posts_path %>
</erb>

Again, as with the +new+ action, the +edit+ action is using the +form+ partial,
this time however, the form will do a PUT action to the PostsController and the
submit button will display "Update Post"

Submitting the form created by this view will invoke the +update+ action within
the controller:

<ruby>
def update
  @post = Post.find(params[:id])

  respond_to do |format|
    if @post.update_attributes(params[:post])
      format.html  { redirect_to(@post,
                    :notice => 'Post was successfully updated.') }
      format.json  { render :json => {}, :status => :ok }
    else
      format.html  { render :action => "edit" }
      format.json  { render :json => @post.errors,
                    :status => :unprocessable_entity }
    end
  end
end
</ruby>

In the +update+ action, Rails first uses the +:id+ parameter passed back from
the edit view to locate the database record that's being edited. The
+update_attributes+ call then takes the rest of the parameters from the request
and applies them to this record. If all goes well, the user is redirected to the
post's +show+ view. If there are any problems, it's back to the +edit+ view to
correct them.

h4. Destroying a Post

Finally, clicking one of the +destroy+ links sends the associated id to the
+destroy+ action:

<ruby>
def destroy
  @post = Post.find(params[:id])
  @post.destroy

  respond_to do |format|
    format.html  { redirect_to(posts_url) }
    format.json  { render :json => {}, :status => :ok }
  end
end
</ruby>

The +destroy+ method of an Active Record model instance removes the
corresponding record from the database. After that's done, there isn't any
record to display, so Rails redirects the user's browser to the index view for
the model.

h3. Adding a Second Model

Now that you've seen how a model built with scaffolding looks like, it's time to
add a second model to the application. The second model will handle comments on<commit_msg>Slight change to reflect current 'destroy' code generated by scaffold in 3.1 release.<commit_after>h2. Getting Started with Rails

This guide covers getting up and running with Ruby on Rails. After reading it,
you should be familiar with:

* Installing Rails, creating a new Rails application, and connecting your application to a database
* The general layout of a Rails application
* The basic principles of MVC (Model, View Controller) and RESTful design
* How to quickly generate the starting pieces of a Rails application

endprologue.

WARNING. This Guide is based on Rails 3.1. Some of the code shown here will not
work in earlier versions of Rails.

h3. Guide Assumptions

This guide is designed for beginners who want to get started with a Rails
application from scratch. It does not assume that you have any prior experience
with Rails. However, to get the most out of it, you need to have some
prerequisites installed:

* The "Ruby":http://www.ruby-lang.org/en/downloads language version 1.8.7 or higher

TIP: Note that Ruby 1.8.7 p248 and p249 have marshaling bugs that crash Rails
3.0. Ruby Enterprise Edition have these fixed since release 1.8.7-2010.02
though. On the 1.9 front, Ruby 1.9.1 is not usable because it outright segfaults
on Rails 3.0, so if you want to use Rails 3 with 1.9.x jump on 1.9.2 for smooth
sailing.

* The "RubyGems":http://rubyforge.org/frs/?group_id=126 packaging system
  ** If you want to learn more about RubyGems, please read the "RubyGems User Guide":http://docs.rubygems.org/read/book/1
* A working installation of the "SQLite3 Database":http://www.sqlite.org

Rails is a web application framework running on the Ruby programming language.
If you have no prior experience with Ruby, you will find a very steep learning
curve diving straight into Rails. There are some good free resources on the
internet for learning Ruby, including:

* "Mr. Neighborly's Humble Little Ruby Book":http://www.humblelittlerubybook.com
* "Programming Ruby":http://www.ruby-doc.org/docs/ProgrammingRuby/
* "Why's (Poignant) Guide to Ruby":http://mislav.uniqpath.com/poignant-guide/

h3. What is Rails?

Rails is a web application development framework written in the Ruby language.
It is designed to make programming web applications easier by making assumptions
about what every developer needs to get started. It allows you to write less
code while accomplishing more than many other languages and frameworks.
Experienced Rails developers also report that it makes web application
development more fun.

Rails is opinionated software. It makes the assumption that there is a "best"
way to do things, and it's designed to encourage that way - and in some cases to
discourage alternatives. If you learn "The Rails Way" you'll probably discover a
tremendous increase in productivity. If you persist in bringing old habits from
other languages to your Rails development, and trying to use patterns you
learned elsewhere, you may have a less happy experience.

The Rails philosophy includes several guiding principles:

* DRY - "Don't Repeat Yourself" - suggests that writing the same code over and over again is a bad thing.
* Convention Over Configuration - means that Rails makes assumptions about what you want to do and how you're going to
do it, rather than requiring you to specify every little thing through endless configuration files.
* REST is the best pattern for web applications - organizing your application around resources and standard HTTP verbs
is the fastest way to go.

h4. The MVC Architecture

At the core of Rails is the Model, View, Controller architecture, usually just
called MVC. MVC benefits include:

* Isolation of business logic from the user interface
* Ease of keeping code DRY
* Making it clear where different types of code belong for easier maintenance

h5. Models

A model represents the information (data) of the application and the rules to
manipulate that data. In the case of Rails, models are primarily used for
managing the rules of interaction with a corresponding database table. In most
cases, each table in your database will correspond to one model in your
application. The bulk of your application's business logic will be concentrated
in the models.

h5. Views

Views represent the user interface of your application. In Rails, views are
often HTML files with embedded Ruby code that perform tasks related solely to
the presentation of the data. Views handle the job of providing data to the web
browser or other tool that is used to make requests from your application.

h5. Controllers

Controllers provide the "glue" between models and views. In Rails, controllers
are responsible for processing the incoming requests from the web browser,
interrogating the models for data, and passing that data on to the views for
presentation.

h4. The Components of Rails

Rails ships as many individual components.  Each of these components are briefly
explained below.  If you are new to Rails, as you read this section, don't get
hung up on the details of each component, as they will be explained in further
detail later.  For instance, we will bring up Rack applications, but you don't
need to know anything about them to continue with this guide.

* Action Pack
  ** Action Controller
  ** Action Dispatch
  ** Action View
* Action Mailer
* Active Model
* Active Record
* Active Resource
* Active Support
* Railties

h5. Action Pack

Action Pack is a single gem that contains Action Controller, Action View and
Action Dispatch. The "VC" part of "MVC".

h6. Action Controller

Action Controller is the component that manages the controllers in a Rails
application. The Action Controller framework processes incoming requests to a
Rails application, extracts parameters, and dispatches them to the intended
action.  Services provided by Action Controller include session management,
template rendering, and redirect management.

h6. Action View

Action View manages the views of your Rails application. It can create both HTML
and XML output by default. Action View manages rendering templates, including
nested and partial templates, and includes built-in AJAX support.  View
templates are covered in more detail in another guide called "Layouts and
Rendering":layouts_and_rendering.html.

h6. Action Dispatch

Action Dispatch handles routing of web requests and dispatches them as you want,
either to your application or any other Rack application.  Rack applications are
a more advanced topic and are covered in a separate guide called "Rails on
Rack":rails_on_rack.html.

h5. Action Mailer

Action Mailer is a framework for building e-mail services. You can use Action
Mailer to receive and process incoming email and send simple plain text or
complex multipart emails based on flexible templates.

h5. Active Model

Active Model provides a defined interface between the Action Pack gem services
and Object Relationship Mapping gems such as Active Record. Active Model allows
Rails to utilize other ORM frameworks in place of Active Record if your
application needs this.

h5. Active Record

Active Record is the base for the models in a Rails application. It provides
database independence, basic CRUD functionality, advanced finding capabilities,
and the ability to relate models to one another, among other services.

h5. Active Resource

Active Resource provides a framework for managing the connection between
business objects and RESTful web services. It implements a way to map web-based
resources to local objects with CRUD semantics.

h5. Active Support

Active Support is an extensive collection of utility classes and standard Ruby
library extensions that are used in Rails, both by the core code and by your
applications.

h5. Railties

Railties is the core Rails code that builds new Rails applications and glues the
various frameworks and plugins together in any Rails application.

h4. REST

Rest stands for Representational State Transfer and is the foundation of the
RESTful architecture. This is generally considered to be Roy Fielding's doctoral
thesis, "Architectural Styles and the Design of Network-based Software
Architectures":http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm. While
you can read through the thesis, REST in terms of Rails boils down to two main
principles:

* Using resource identifiers such as URLs to represent resources.
* Transferring representations of the state of that resource between system components.

For example, the following HTTP request:

<tt>DELETE /photos/17</tt>

would be understood to refer to a photo resource with the ID of 17, and to
indicate a desired action - deleting that resource. REST is a natural style for
the architecture of web applications, and Rails hooks into this shielding you
from many of the RESTful complexities and browser quirks.

If you'd like more details on REST as an architectural style, these resources
are more approachable than Fielding's thesis:

* "A Brief Introduction to REST":http://www.infoq.com/articles/rest-introduction by <NAME>
* "An Introduction to REST":http://bitworking.org/news/373/An-Introduction-to-REST (video tutorial) by <NAME>
* "Representational State Transfer":http://en.wikipedia.org/wiki/Representational_State_Transfer article in Wikipedia
* "How to GET a Cup of Coffee":http://www.infoq.com/articles/webber-rest-workflow by <NAME>, <NAME> &
<NAME>

h3. Creating a New Rails Project

If you follow this guide, you'll create a Rails project called <tt>blog</tt>, a
(very) simple weblog. Before you can start building the application, you need to
make sure that you have Rails itself installed.

TIP: The examples below use # and $ to denote terminal prompts. If you are using Windows, your prompt will look something like c:\source_code>

h4. Installing Rails

In most cases, the easiest way to install Rails is to take advantage of RubyGems:

<shell>
Usually run this as the root user:
# gem install rails
</shell>

TIP. If you're working on Windows, you can quickly install Ruby and Rails with
"Rails Installer":http://railsinstaller.org.

h4. Creating the Blog Application

The best way to use this guide is to follow each step as it happens, no code or
step needed to make this example application has been left out, so you can
literally follow along step by step. If you need to see the completed code, you
can download it from "Getting Started
Code":https://github.com/mikel/getting-started-code.

To begin, open a terminal, navigate to a folder where you have rights to create
files, and type:

<shell>
$ rails new blog
</shell>

This will create a Rails application called Blog in a directory called blog.

TIP: You can see all of the switches that the Rails application builder accepts
by running
<tt>rails new -h</tt>.

After you create the blog application, switch to its folder to continue work
directly in that application:

<shell>
$ cd blog
</shell>

In any case, Rails will create a folder in your working directory called
<tt>blog</tt>. Open up that folder and explore its contents. Most of the work in
this tutorial will happen in the <tt>app/</tt> folder, but here's a basic
rundown on the function of each folder that Rails creates in a new application
by default:

|_.File/Folder|_.Purpose|
|Gemfile|This file allows you to specify what gem dependencies are needed for your Rails application. See section on Bundler, below.|
|README|This is a brief instruction manual for your application. You should edit this file to tell others what your application does, how to set it up, and so on.|
|Rakefile|This file locates and loads tasks that can be run from the command line. The task definitions are defined throughout the components of Rails. Rather than changing Rakefile, you should add your own tasks by adding files to the lib/tasks directory of your application.|
|app/|Contains the controllers, models, views and assets for your application. You'll focus on this folder for the remainder of this guide.|
|config/|Configure your application's runtime rules, routes, database, and more.|
|config.ru|Rack configuration for Rack based servers used to start the application.|
|db/|Shows your current database schema, as well as the database migrations. You'll learn about migrations shortly.|
|doc/|In-depth documentation for your application.|
|lib/|Extended modules for your application (not covered in this guide).|
|log/|Application log files.|
|public/|The only folder seen to the world as-is. Contains the static files and compiled assets.|
|script/|Contains the rails script that starts your app and can contain other scripts you use to deploy or run your application.|
|test/|Unit tests, fixtures, and other test apparatus. These are covered in "Testing Rails Applications":testing.html|
|tmp/|Temporary files|
|vendor/|A place for all third-party code. In a typical Rails application, this includes Ruby Gems, the Rails source code (if you install it into your project) and plugins containing additional prepackaged functionality.|

h4. Configuring a Database

Just about every Rails application will interact with a database. The database
to use is specified in a configuration file, +config/database.yml+.  If you open
this file in a new Rails application, you'll see a default database
configuration using SQLite3. The file contains sections for three different
environments in which Rails can run by default:

* The +development+ environment is used on your development computer as you interact manually with the application.
* The +test+ environment is used to run automated tests.
* The +production+ environment is used when you deploy your application for the world to use.

h5. Configuring an SQLite3 Database

Rails comes with built-in support for "SQLite3":http://www.sqlite.org, which is
a lightweight serverless database application. While a busy production
environment may overload SQLite, it works well for development and testing.
Rails defaults to using an SQLite database when creating a new project, but you
can always change it later.

Here's the section of the default configuration file
(<tt>config/database.yml</tt>) with connection information for the development
environment:

<yaml>
development:
  adapter: sqlite3
  database: db/development.sqlite3
  pool: 5
  timeout: 5000
</yaml>

NOTE: In this guide we are using an SQLite3 database for data storage, because
it is a zero configuration database that just works. Rails also supports MySQL
and PostgreSQL "out of the box", and has plugins for many database systems. If
you are using a database in a production environment Rails most likely has an
adapter for it.

h5. Configuring a MySQL Database

If you choose to use MySQL instead of the shipped SQLite3 database, your
+config/database.yml+ will look a little different. Here's the development
section:

<yaml>
development:
  adapter: mysql2
  encoding: utf8
  database: blog_development
  pool: 5
  username: root
  password:
  socket: /tmp/mysql.sock
</yaml>

If your development computer's MySQL installation includes a root user with an
empty password, this configuration should work for you. Otherwise, change the
username and password in the +development+ section as appropriate.

h5. Configuring a PostgreSQL Database

If you choose to use PostgreSQL, your +config/database.yml+ will be customized
to use PostgreSQL databases:

<yaml>
development:
  adapter: postgresql
  encoding: unicode
  database: blog_development
  pool: 5
  username: blog
  password:
</yaml>

h5. Configuring an SQLite3 Database for JRuby Platform

If you choose to use SQLite3 and using JRuby, your +config/database.yml+ will
look a little different. Here's the development section:

<yaml>
development:
  adapter: jdbcsqlite3
  database: db/development.sqlite3
</yaml>

h5. Configuring a MySQL Database for JRuby Platform

If you choose to use MySQL and using JRuby, your +config/database.yml+ will look
a little different. Here's the development section:

<yaml>
development:
  adapter: jdbcmysql
  database: blog_development
  username: root
  password:
</yaml>

h5. Configuring a PostgreSQL Database for JRuby Platform

Finally if you choose to use PostgreSQL and using JRuby, your
+config/database.yml+ will look a little different. Here's the development
section:

<yaml>
development:
  adapter: jdbcpostgresql
  encoding: unicode
  database: blog_development
  username: blog
  password:
</yaml>

Change the username and password in the +development+ section as appropriate.

TIP: You don't have to update the database configurations manually. If you look at the
options of the application generator, you will see that one of the options
is named <tt>--database</tt>. This option allows you to choose an adapter from a
list of the most used relational databases. You can even run the generator
repeatedly: <tt>cd .. && rails new blog --database=mysql</tt>. When you confirm the overwriting
 of the +config/database.yml+ file, your application will be configured for MySQL
instead of SQLite.

h4. Creating the Database

Now that you have your database configured, it's time to have Rails create an
empty database for you. You can do this by running a rake command:

<shell>
$ rake db:create
</shell>

This will create your development and test SQLite3 databases inside the
<tt>db/</tt> folder.

TIP: Rake is a general-purpose command-runner that Rails uses for many things.
You can see the list of available rake commands in your application by running
+rake -T+.

h3. Hello, Rails!

One of the traditional places to start with a new language is by getting some
text up on screen quickly. To do this, you need to get your Rails application
server running.

h4. Starting up the Web Server

You actually have a functional Rails application already. To see it, you need to
start a web server on your development machine. You can do this by running:

<shell>
$ rails server
</shell>

This will fire up an instance of the WEBrick web server by default (Rails can
also use several other web servers). To see your application in action, open a
browser window and navigate to "http://localhost:3000":http://localhost:3000.
You should see Rails' default information page:

!images/rails_welcome.png(Welcome Aboard screenshot)!

TIP: To stop the web server, hit Ctrl+C in the terminal window where it's
running. In development mode, Rails does not generally require you to stop the
server; changes you make in files will be automatically picked up by the server.

The "Welcome Aboard" page is the _smoke test_ for a new Rails application: it
makes sure that you have your software configured correctly enough to serve a
page. You can also click on the _About your application’s environment_ link to
see a summary of your application's environment.

h4. Say "Hello", Rails

To get Rails saying "Hello", you need to create at minimum a controller and a
view. Fortunately, you can do that in a single command. Enter this command in
your terminal:

<shell>
$ rails generate controller home index
</shell>

TIP: If you get a command not found error when running this command, you
need to explicitly pass Rails +rails+ commands to Ruby: <tt>ruby
\path\to\your\application\script\rails generate controller home index</tt>.

Rails will create several files for you, including
+app/views/home/index.html.erb+. This is the template that will be used to
display the results of the +index+ action (method) in the +home+ controller.
Open this file in your text editor and edit it to contain a single line of code:

<code class="html">
<h1>Hello, Rails!</h1>
</code>

h4. Setting the Application Home Page

Now that we have made the controller and view, we need to tell Rails when we
want "Hello Rails" to show up. In our case, we want it to show up when we
navigate to the root URL of our site,
"http://localhost:3000":http://localhost:3000, instead of the "Welcome Aboard"
smoke test.

The first step to doing this is to delete the default page from your
application:

<shell>
$ rm public/index.html
</shell>

We need to do this as Rails will deliver any static file in the +public+
directory in preference to any dynamic content we generate from the controllers.

Now, you have to tell Rails where your actual home page is located. Open the
file +config/routes.rb+ in your editor. This is your application's _routing
file_ which holds entries in a special DSL (domain-specific language) that tells
Rails how to connect incoming requests to controllers and actions. This file
contains many sample routes on commented lines, and one of them actually shows
you how to connect the root of your site to a specific controller and action.
Find the line beginning with +root :to+, uncomment it and change it like the
following:

<ruby>
Blog::Application.routes.draw do

  #...
  # You can have the root of your site routed with "root"
  # just remember to delete public/index.html.
  root :to => "home#index"
</ruby>

The +root :to => "home#index"+ tells Rails to map the root action to the home
controller's index action.

Now if you navigate to "http://localhost:3000":http://localhost:3000 in your
browser, you'll see +Hello, Rails!+.

NOTE. For more information about routing, refer to "Rails Routing from the
Outside In":routing.html.

h3. Getting Up and Running Quickly with Scaffolding

Rails _scaffolding_ is a quick way to generate some of the major pieces of an
application. If you want to create the models, views, and controllers for a new
resource in a single operation, scaffolding is the tool for the job.

h3. Creating a Resource

In the case of the blog application, you can start by generating a scaffolded
Post resource: this will represent a single blog posting. To do this, enter this
command in your terminal:

<shell>
$ rails generate scaffold Post name:string title:string content:text
</shell>

The scaffold generator will build several files in your application, along with some
folders, and edit <tt>config/routes.rb</tt>. Here's a quick overview of what it creates:

|_.File                                       |_.Purpose|
|db/migrate/20100207214725_create_posts.rb    |Migration to create the posts table in your database (your name will include a different timestamp)|
|app/models/post.rb                           |The Post model|
|test/fixtures/posts.yml                      |Dummy posts for use in testing|
|app/controllers/posts_controller.rb          |The Posts controller|
|app/views/posts/index.html.erb               |A view to display an index of all posts |
|app/views/posts/edit.html.erb                |A view to edit an existing post|
|app/views/posts/show.html.erb                |A view to display a single post|
|app/views/posts/new.html.erb                 |A view to create a new post|
|app/views/posts/_form.html.erb               |A partial to control the overall look and feel of the form used in edit and new views|
|app/helpers/posts_helper.rb                  |Helper functions to be used from the post views|
|app/assets/stylesheets/scaffolds.css.scss    |Cascading style sheet to make the scaffolded views look better|
|app/assets/stylesheets/posts.css.scss        |Cascading style sheet for the posts controller|
|app/assets/javascripts/posts.js.coffee       |CoffeeScript for the posts controller|
|test/unit/post_test.rb                       |Unit testing harness for the posts model|
|test/functional/posts_controller_test.rb     |Functional testing harness for the posts controller|
|test/unit/helpers/posts_helper_test.rb       |Unit testing harness for the posts helper|
|config/routes.rb                             |Edited to include routing information for posts|

NOTE. While scaffolding will get you up and running quickly, the code it
generates is unlikely to be a perfect fit for your application. You'll most
probably want to customize the generated code. Many experienced Rails developers
avoid scaffolding entirely, preferring to write all or most of their source code
from scratch. Rails, however, makes it really simple to customize templates for
generated models, controllers, views and other source files. You'll find more
information in the "Creating and Customizing Rails Generators &
Templates":generators.html guide.

h4. Running a Migration

One of the products of the +rails generate scaffold+ command is a _database
migration_. Migrations are Ruby classes that are designed to make it simple to
create and modify database tables. Rails uses rake commands to run migrations,
and it's possible to undo a migration after it's been applied to your database.
Migration filenames include a timestamp to ensure that they're processed in the
order that they were created.

If you look in the +db/migrate/20100207214725_create_posts.rb+ file (remember,
yours will have a slightly different name), here's what you'll find:

<ruby>
class CreatePosts < ActiveRecord::Migration
  def change
    create_table :posts do |t|
      t.string :name
      t.string :title
      t.text :content

      t.timestamps
    end
  end
end
</ruby>

The above migration creates a method named +change+ which will be called when you
run this migration. The action defined in that method is also reversible, which
means Rails knows how to reverse the change made by this migration, in case you
want to reverse it at later date. By default, when you run this migration it
will creates a +posts+ table with two string columns and a text column. It also
creates two timestamp fields to track record creation and updating. More
information about Rails migrations can be found in the "Rails Database
Migrations":migrations.html guide.

At this point, you can use a rake command to run the migration:

<shell>
$ rake db:migrate
</shell>

Rails will execute this migration command and tell you it created the Posts
table.

<shell>
==  CreatePosts: migrating ====================================================
-- create_table(:posts)
   -> 0.0019s
==  CreatePosts: migrated (0.0020s) ===========================================
</shell>

NOTE. Because you're working in the development environment by default, this
command will apply to the database defined in the +development+ section of your
+config/database.yml+ file. If you would like to execute migrations in other
environment, for instance in production, you must explicitly pass it when
invoking the command: <tt>rake db:migrate RAILS_ENV=production</tt>.

h4. Adding a Link

To hook the posts up to the home page you've already created, you can add a link
to the home page. Open +app/views/home/index.html.erb+ and modify it as follows:

<ruby>
<h1>Hello, Rails!</h1>
<%= link_to "My Blog", posts_path %>
</ruby>

The +link_to+ method is one of Rails' built-in view helpers. It creates a
hyperlink based on text to display and where to go - in this case, to the path
for posts.

h4. Working with Posts in the Browser

Now you're ready to start working with posts. To do that, navigate to
"http://localhost:3000":http://localhost:3000/ and then click the "My Blog"
link:

!images/posts_index.png(Posts Index screenshot)!

This is the result of Rails rendering the +index+ view of your posts. There
aren't currently any posts in the database, but if you click the +New Post+ link
you can create one. After that, you'll find that you can edit posts, look at
their details, or destroy them. All of the logic and HTML to handle this was
built by the single +rails generate scaffold+ command.

TIP: In development mode (which is what you're working in by default), Rails
reloads your application with every browser request, so there's no need to stop
and restart the web server.

Congratulations, you're riding the rails! Now it's time to see how it all works.

h4. The Model

The model file, +app/models/post.rb+ is about as simple as it can get:

<ruby>
class Post < ActiveRecord::Base
end
</ruby>

There isn't much to this file - but note that the +Post+ class inherits from
+ActiveRecord::Base+. Active Record supplies a great deal of functionality to
your Rails models for free, including basic database CRUD (Create, Read, Update,
Destroy) operations, data validation, as well as sophisticated search support
and the ability to relate multiple models to one another.

h4. Adding Some Validation

Rails includes methods to help you validate the data that you send to models.
Open the +app/models/post.rb+ file and edit it:

<ruby>
class Post < ActiveRecord::Base
  validates :name,  :presence => true
  validates :title, :presence => true,
                    :length => { :minimum => 5 }
end
</ruby>

These changes will ensure that all posts have a name and a title, and that the
title is at least five characters long. Rails can validate a variety of
conditions in a model, including the presence or uniqueness of columns, their
format, and the existence of associated objects.

h4. Using the Console

To see your validations in action, you can use the console. The console is a
command-line tool that lets you execute Ruby code in the context of your
application:

<shell>
$ rails console
</shell>

TIP: The default console will make changes to your database. You can instead
open a console that will roll back any changes you make by using +rails console
--sandbox+.

After the console loads, you can use it to work with your application's models:

<shell>
>> p = Post.new(:content => "A new post")
=> #<Post id: nil, name: nil, title: nil,
     content: "A new post", created_at: nil,
     updated_at: nil>
>> p.save
=> false
>> p.errors
=> #<OrderedHash { :title=>["can't be blank",
                           "is too short (minimum is 5 characters)"],
                   :name=>["can't be blank"] }>
</shell>

This code shows creating a new +Post+ instance, attempting to save it and
getting +false+ for a return value (indicating that the save failed), and
inspecting the +errors+ of the post.

When you're finished, type +exit+ and hit +return+ to exit the console.

TIP: Unlike the development web server, the console does not automatically load
your code afresh for each line. If you make changes to your models while the
console is open, type +reload!+ at the console prompt to load them.

h4. Listing All Posts

The easiest place to start looking at functionality is with the code that lists
all posts. Open the file +app/controllers/posts_controller.rb+ and look at the
+index+ action:

<ruby>
def index
  @posts = Post.all

  respond_to do |format|
    format.html  # index.html.erb
    format.json  { render :json => @posts }
  end
end
</ruby>

+Post.all+ calls the +Post+ model to return all of the posts currently in the
database. The result of this call is an array of posts that we store in an
instance variable called +@posts+.

TIP: For more information on finding records with Active Record, see "Active
Record Query Interface":active_record_querying.html.

The +respond_to+ block handles both HTML and JSON calls to this action. If you
browse to "http://localhost:3000/posts.json":http://localhost:3000/posts.json,
you'll see a JSON containing all of the posts. The HTML format looks for a view
in +app/views/posts/+ with a name that corresponds to the action name. Rails
makes all of the instance variables from the action available to the view.
Here's +app/views/posts/index.html.erb+:

<erb>
<h1>Listing posts</h1>

<table>
  <tr>
    <th>Name</th>
    <th>Title</th>
    <th>Content</th>
    <th></th>
    <th></th>
    <th></th>
  </tr>

<% @posts.each do |post| %>
  <tr>
    <td><%= post.name %></td>
    <td><%= post.title %></td>
    <td><%= post.content %></td>
    <td><%= link_to 'Show', post %></td>
    <td><%= link_to 'Edit', edit_post_path(post) %></td>
    <td><%= link_to 'Destroy', post, :confirm => 'Are you sure?', :method => :delete %></td>
  </tr>
<% end %>
</table>

<br />

<%= link_to 'New post', new_post_path %>
</erb>

This view iterates over the contents of the +@posts+ array to display content
and links. A few things to note in the view:

* +link_to+ builds a hyperlink to a particular destination
* +edit_post_path+ and +new_post_path+ are helpers that Rails provides as part of RESTful routing. You'll see a variety of these helpers for the different actions that the controller includes.

NOTE. In previous versions of Rails, you had to use +&lt;%=h post.name %&gt;+ so
that any HTML would be escaped before being inserted into the page. In Rails
3.0, this is now the default. To get unescaped HTML, you now use +&lt;%= raw
post.name %&gt;+.

TIP: For more details on the rendering process, see "Layouts and Rendering in
Rails":layouts_and_rendering.html.

h4. Customizing the Layout

The view is only part of the story of how HTML is displayed in your web browser.
Rails also has the concept of +layouts+, which are containers for views. When
Rails renders a view to the browser, it does so by putting the view's HTML into
a layout's HTML. In previous versions of Rails, the +rails generate scaffold+
command would automatically create a controller specific layout, like
+app/views/layouts/posts.html.erb+, for the posts controller. However this has
been changed in Rails 3.0. An application specific +layout+ is used for all the
controllers and can be found in +app/views/layouts/application.html.erb+. Open
this layout in your editor and modify the +body+ tag:

<erb>
<!DOCTYPE html>
<html>
<head>
  <title>Blog</title>
  <%= stylesheet_link_tag "application" %>
  <%= javascript_include_tag "application" %>
  <%= csrf_meta_tags %>
</head>
<body style="background: #EEEEEE;">

<%= yield %>

</body>
</html>
</erb>

Now when you refresh the +/posts+ page, you'll see a gray background to the
page. This same gray background will be used throughout all the views for posts.

h4. Creating New Posts

Creating a new post involves two actions. The first is the +new+ action, which
instantiates an empty +Post+ object:

<ruby>
def new
  @post = Post.new

  respond_to do |format|
    format.html  # new.html.erb
    format.json  { render :json => @post }
  end
end
</ruby>

The +new.html.erb+ view displays this empty Post to the user:

<erb>
<h1>New post</h1>

<%= render 'form' %>

<%= link_to 'Back', posts_path %>
</erb>

The +&lt;%= render 'form' %&gt;+ line is our first introduction to _partials_ in
Rails. A partial is a snippet of HTML and Ruby code that can be reused in
multiple locations. In this case, the form used to make a new post, is basically
identical to a form used to edit a post, both have text fields for the name and
title and a text area for the content with a button to make a new post or update
the existing post.

If you take a look at +views/posts/_form.html.erb+ file, you will see the
following:

<erb>
<%= form_for(@post) do |f| %>
  <% if @post.errors.any? %>
  <div id="errorExplanation">
    <h2><%= pluralize(@post.errors.count, "error") %> prohibited this post from being saved:</h2>
    <ul>
    <% @post.errors.full_messages.each do |msg| %>
      <li><%= msg %></li>
    <% end %>
    </ul>
  </div>
  <% end %>

  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>
  <div class="field">
    <%= f.label :title %><br />
    <%= f.text_field :title %>
  </div>
  <div class="field">
    <%= f.label :content %><br />
    <%= f.text_area :content %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>
</erb>

This partial receives all the instance variables defined in the calling view
file, so in this case, the controller assigned the new Post object to +@post+
and so, this is available in both the view and partial as +@post+.

For more information on partials, refer to the "Layouts and Rendering in
Rails":layouts_and_rendering.html#using-partials guide.

The +form_for+ block is used to create an HTML form. Within this block, you have
access to methods to build various controls on the form. For example,
+f.text_field :name+ tells Rails to create a text input on the form, and to hook
it up to the +name+ attribute of the instance being displayed. You can only use
these methods with attributes of the model that the form is based on (in this
case +name+, +title+, and +content+). Rails uses +form_for+ in preference to
having you write raw HTML because the code is more succinct, and because it
explicitly ties the form to a particular model instance.

The +form_for+ block is also smart enough to work out if you are doing a _New
Post_ or an _Edit Post_ action, and will set the form +action+ tags and submit
button names appropriately in the HTML output.

TIP: If you need to create an HTML form that displays arbitrary fields, not tied
to a model, you should use the +form_tag+ method, which provides shortcuts for
building forms that are not necessarily tied to a model instance.

When the user clicks the +Create Post+ button on this form, the browser will
send information back to the +create+ method of the controller (Rails knows to
call the +create+ method because the form is sent with an HTTP POST request;
that's one of the conventions that I mentioned earlier):

<ruby>
def create
  @post = Post.new(params[:post])

  respond_to do |format|
    if @post.save
      format.html  { redirect_to(@post,
                    :notice => 'Post was successfully created.') }
      format.json  { render :json => @post,
                    :status => :created, :location => @post }
    else
      format.html  { render :action => "new" }
      format.json  { render :json => @post.errors,
                    :status => :unprocessable_entity }
    end
  end
end
</ruby>

The +create+ action instantiates a new Post object from the data supplied by the
user on the form, which Rails makes available in the +params+ hash. After
successfully saving the new post, +create+ returns the appropriate format that
the user has requested (HTML in our case). It then redirects the user to the
resulting post +show+ action and sets a notice to the user that the Post was
successfully created.

If the post was not successfully saved, due to a validation error, then the
controller returns the user back to the +new+ action with any error messages so
that the user has the chance to fix the error and try again.

The "Post was successfully created." message is stored inside of the Rails
+flash+ hash, (usually just called _the flash_) so that messages can be carried
over to another action, providing the user with useful information on the status
of their request. In the case of +create+, the user never actually sees any page
rendered during the Post creation process, because it immediately redirects to
the new Post as soon Rails saves the record. The Flash carries over a message to
the next action, so that when the user is redirected back to the +show+ action,
they are presented with a message saying "Post was successfully created."

h4. Showing an Individual Post

When you click the +show+ link for a post on the index page, it will bring you
to a URL like +http://localhost:3000/posts/1+. Rails interprets this as a call
to the +show+ action for the resource, and passes in +1+ as the +:id+ parameter.
Here's the +show+ action:

<ruby>
def show
  @post = Post.find(params[:id])

  respond_to do |format|
    format.html  # show.html.erb
    format.json  { render :json => @post }
  end
end
</ruby>

The +show+ action uses +Post.find+ to search for a single record in the database
by its id value. After finding the record, Rails displays it by using
+show.html.erb+:

<erb>
<p class="notice"><%= notice %></p>

<p>
  <b>Name:</b>
  <%= @post.name %>
</p>

<p>
  <b>Title:</b>
  <%= @post.title %>
</p>

<p>
  <b>Content:</b>
  <%= @post.content %>
</p>


<%= link_to 'Edit', edit_post_path(@post) %> |
<%= link_to 'Back', posts_path %>
</erb>

h4. Editing Posts

Like creating a new post, editing a post is a two-part process. The first step
is a request to +edit_post_path(@post)+ with a particular post. This calls the
+edit+ action in the controller:

<ruby>
def edit
  @post = Post.find(params[:id])
end
</ruby>

After finding the requested post, Rails uses the +edit.html.erb+ view to display
it:

<erb>
<h1>Editing post</h1>

<%= render 'form' %>

<%= link_to 'Show', @post %> |
<%= link_to 'Back', posts_path %>
</erb>

Again, as with the +new+ action, the +edit+ action is using the +form+ partial,
this time however, the form will do a PUT action to the PostsController and the
submit button will display "Update Post"

Submitting the form created by this view will invoke the +update+ action within
the controller:

<ruby>
def update
  @post = Post.find(params[:id])

  respond_to do |format|
    if @post.update_attributes(params[:post])
      format.html  { redirect_to(@post,
                    :notice => 'Post was successfully updated.') }
      format.json  { render :json => {}, :status => :ok }
    else
      format.html  { render :action => "edit" }
      format.json  { render :json => @post.errors,
                    :status => :unprocessable_entity }
    end
  end
end
</ruby>

In the +update+ action, Rails first uses the +:id+ parameter passed back from
the edit view to locate the database record that's being edited. The
+update_attributes+ call then takes the rest of the parameters from the request
and applies them to this record. If all goes well, the user is redirected to the
post's +show+ view. If there are any problems, it's back to the +edit+ view to
correct them.

h4. Destroying a Post

Finally, clicking one of the +destroy+ links sends the associated id to the
+destroy+ action:

<ruby>
def destroy
  @post = Post.find(params[:id])
  @post.destroy

  respond_to do |format|
    format.html { redirect_to posts_url }
    format.json { head :ok }
  end
end
</ruby>

The +destroy+ method of an Active Record model instance removes the
corresponding record from the database. After that's done, there isn't any
record to display, so Rails redirects the user's browser to the index view for
the model.

h3. Adding a Second Model

Now that you've seen how a model built with scaffolding looks like, it's time to
add a second model to the application. The second model will handle comments on
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