path: root/railties/guides/source/getting_started.textile

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.

WARNING: The Edge version of this guide is currently being re-worked. Please excuse us while we re-arrange the place.

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?

TIP: This section goes into the background and philosophy of the Rails framework
in detail. You can safely skip this section and come back to it at a later time.
Section 3 starts you on the path to creating your first Rails application.

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 two major 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.

h3. Creating a New Rails Project

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. You can get the complete code "here":https://github.com/lifo/docrails/tree/master/railties/guides/code/getting_started.

By following along with 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

To install Rails, use the +gem install+ command provided by RubyGems:

<shell>
# gem install rails
</shell>

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

To verify that you have everything installed correctly, you should be able to run the following:

<shell>
$ rails --version
</shell>

If it says something like "Rails 3.2.2" you are ready to continue.

h4. Creating the Blog Application

Rails comes with a number of generators that are designed to make your development life easier. One of these is the new application generator, which will provide you with the foundation of a Rails application so that you don't have to write it yourself.

To use this generator, open a terminal, navigate to a directory 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>

The +rails new blog+ command we ran above created a folder in your working directory called <tt>blog</tt>. The <tt>blog</tt> directory has a number of auto-generated folders that make up the structure of a Rails application. 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 of the files and folders that Rails created by default:

|_.File/Folder|_.Purpose|
|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.  This is covered in more detail in "Configuring Rails Applications":configuring.html|
|config.ru|Rack configuration for Rack based servers used to start the application.|
|db/|Contains your current database schema, as well as the database migrations.|
|doc/|In-depth documentation for your application.|
|Gemfile<BR />Gemfile.lock|These files allow you to specify what gem dependencies are needed for your Rails application. These files are used by the Bundler gem. For more information about Bundler, see "the Bundler website":http://gembundler.com |
|lib/|Extended modules for your application.|
|log/|Application log files.|
|public/|The only folder seen to the world as-is. Contains the static files and compiled assets.|
|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.|
|README.rdoc|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.|
|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 and the Rails source code (if you optionally install it into your project).|

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>

TIP: Compiling CoffeeScript to JavaScript requires a JavaScript runtime and the absence of a runtime will give you an +execjs+ error. Usually Mac OS X and Windows come with a JavaScript runtime installed. Rails adds the +therubyracer+ gem to Gemfile in a commented line for new apps and you can uncomment if you need it. +therubyrhino+ is the recommended runtime for JRuby users and is added by default to Gemfile in apps generated under JRuby. You can investigate about all the supported runtimes at "ExecJS":https://github.com/sstephenson/execjs#readme.

This will fire up an instance of a webserver built into Ruby called WEBrick by default. 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_.

A controller's purpose is to receive specific requests for the application. What controller receives what request is determined by the _routing_. There is very often more than one route to each controller, and different routes can be served by different _actions_. Each action's purpose is to collect information to provide it to a view.

A view's purpose is to display this information in a human readable format. An important distinction to make is that it is the _controller_, not the view, where information is collected. The view should just display that information. By default, view templates are written in a language called ERB (Embedded Ruby) which is converted by the request cycle in Rails before being sent to the user.

To create a new controller, run this command:

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

Rails will create several files for you. Most important of these are of course the controller, located at +app/controllers/welcome_controller.rb+ and the view, located at +app/views/welcome/index.html.erb+.

Open the +app/views/welcome/index/html.erb+ 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. At the moment, however, the "Welcome Aboard" smoke test is occupying that spot.

To fix this, delete the +index.html+ file located inside the +public+ directory of the application.

We need to do this because Rails will serve any static file in the +public+ directory that matches a route in preference to any dynamic content we generate from the controllers.

Next, 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+ and uncomment it. It should look something 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 => "welcome#index"
</ruby>

The +root :to => "welcome#index"+ tells Rails to map requests to the root of the application to the welcome controller's index action. This was created earlier when you ran the controller generator (+rails generate controller welcome index+).

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

Now that you've seen how to create a controller, an action and a view, let's create something with a bit more substance.

In the Blog application, you will now create a new _resource_. A resource is the term used for a collection of similar objects, such as posts, people or animals. You can create, read, update and destroy items for a resource and these operations are referred to as _CRUD_ operations.

In the next section, you will add the ability to create new posts in your application and be able to view them. This is the "CR" from CRUD. The form for doing this will look like this:

!images/getting_started/new_post.png(The new post form)!

It will look a little basic for now, but that's ok. We'll look at improving the styling for it afterwards.

h4. Creating Posts

The first thing that you are going to need to create a new post within the application is a place to do that. A great place for that would be at +/posts/new+. If you attempt to navigate to that now -- by visiting "http://localhost:3000/posts/new":http://localhost:3000/posts/new -- Rails will give you a routing error:


!images/getting_started/routing_error_no_route_matches.png(A routing error, no route matches /posts/new)!

This is because there is nowhere inside the routes for the application -- defined inside +config/routes.rb+ -- that defines this route. By default, Rails has no routes configured at all, and so you must define your routes as you need them.

 To do this, you're going to need to create a route inside +config/routes.rb+ file, on a new line between the +do+ and the +end+ for the +draw+ method:

<ruby>
get "posts/new"
</ruby>

This route is a super-simple route: it defines a new route that only responds to +GET+ requests, and that the route is at +posts/new+. But how does it know where to go without the use of the +:to+ option? Well, Rails uses a sensible default here: Rails will assume that we want this route to go to the new action inside the posts controller.

With the route defined, requests can now be made to +/posts/new+ in the application. Navigate to "http://localhost:3000/posts/new":http://localhost:3000/posts/new and you'll see another routing error:

!images/getting_started/routing_error_no_controller.png(Another routing error, uninitialized constant PostsController)

This error is happening because this route need a controller to be defined. The route is attempting to find that controller so it can serve the request, but with the controller undefined, it just can't do that.

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 this method is also reversible, which
means Rails knows how to reverse the change made by this migration, in case you
want to reverse it later. When you run this migration it will create a
+posts+ table with two string columns and a text column. It also creates two
timestamp fields to allow Rails to track post creation and update times. 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 another
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/welcome/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. Validations are covered in detail
in "Active Record Validations and Callbacks":active_record_validations_callbacks.html#validations-overview

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 <tt>rails console
--sandbox</tt>.

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.full_messages
=> ["Name can't be blank", "Title can't be blank", "Title is too short (minimum is 5 characters)"]
</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 (in your editor)
while the console is open, type +reload!+ at the console prompt to load them.

h4. Listing All Posts

Let's dive into the Rails code a little deeper to see how the application is
showing us the list of 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+ returns all of the posts currently in the database as an array
of +Post+ records 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 and above, this is now the default. To get unescaped HTML, you now use <tt>&lt;%= raw post.name %&gt;</tt>.

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. 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 to include the style directive
below:

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

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 the form used to edit a post, both having text fields for the name and
title, a text area for the content, and a button to create the new post or to update
the existing one.

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. In this case, the controller assigned the new +Post+ object to +@post+,
which will thus be available in both the view and the 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+ action of the controller (Rails knows to
call the +create+ action because the form is sent with an HTTP POST request;
that's one of the conventions that were 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 in 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 as 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
+app/views/posts/show.html.erb+:

<erb>
<p id="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  { head :no_content }
    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 +post+ parameter (a hash) from the request
and applies it to this record. If all goes well, the user is redirected to the
post's +show+ action. If there are any problems, it redirects back to the +edit+ action 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 :no_content }
  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 action of
the controller.

h3. Adding a Second Model

Now that you've seen what 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
blog posts.

h4. Generating a Model

Models in Rails use a singular name, and their corresponding database tables use
a plural name. For the model to hold comments, the convention is to use the name
+Comment+. Even if you don't want to use the entire apparatus set up by
scaffolding, most Rails developers still use generators to make things like
models and controllers. To create the new model, run this command in your
terminal:

<shell>
$ rails generate model Comment commenter:string body:text post:references
</shell>

This command will generate four files:

|_.File                                       |_.Purpose|
|db/migrate/20100207235629_create_comments.rb | Migration to create the comments table in your database (your name will include a different timestamp) |
| app/models/comment.rb                       | The Comment model |
| test/unit/comment_test.rb                   | Unit testing harness for the comments model |
| test/fixtures/comments.yml                  | Sample comments for use in testing |

First, take a look at +comment.rb+:

<ruby>
class Comment < ActiveRecord::Base
  belongs_to :post
end
</ruby>

This is very similar to the +post.rb+ model that you saw earlier. The difference
is the line +belongs_to :post+, which sets up an Active Record _association_.
You'll learn a little about associations in the next section of this guide.

In addition to the model, Rails has also made a migration to create the
corresponding database table:

<ruby>
class CreateComments < ActiveRecord::Migration
  def change
    create_table :comments do |t|
      t.string :commenter
      t.text :body
      t.references :post

      t.timestamps
    end

    add_index :comments, :post_id
  end
end
</ruby>

The +t.references+ line sets up a foreign key column for the association between
the two models. And the +add_index+ line sets up an index for this association
column. Go ahead and run the migration:

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

Rails is smart enough to only execute the migrations that have not already been
run against the current database, so in this case you will just see:

<shell>
==  CreateComments: migrating =================================================
-- create_table(:comments)
   -> 0.0008s
-- add_index(:comments, :post_id)
   -> 0.0003s
==  CreateComments: migrated (0.0012s) ========================================
</shell>

h4. Associating Models

Active Record associations let you easily declare the relationship between two
models. In the case of comments and posts, you could write out the relationships
this way:

* Each comment belongs to one post.
* One post can have many comments.

In fact, this is very close to the syntax that Rails uses to declare this
association. You've already seen the line of code inside the Comment model that
makes each comment belong to a Post:

<ruby>
class Comment < ActiveRecord::Base
  belongs_to :post
end
</ruby>

You'll need to edit the +post.rb+ file to add the other side of the association:

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

  has_many :comments
end
</ruby>

These two declarations enable a good bit of automatic behavior. For example, if
you have an instance variable +@post+ containing a post, you can retrieve all
the comments belonging to that post as an array using +@post.comments+.

TIP: For more information on Active Record associations, see the "Active Record
Associations":association_basics.html guide.

h4. Adding a Route for Comments

As with the +welcome+ controller, we will need to add a route so that Rails knows
where we would like to navigate to see +comments+. Open up the
+config/routes.rb+ file again. Near the top, you will see the entry for +posts+
that was added automatically by the scaffold generator: <tt>resources
:posts</tt>. Edit it as follows:

<ruby>
resources :posts do
  resources :comments
end
</ruby>

This creates +comments+ as a _nested resource_ within +posts+. This is another
part of capturing the hierarchical relationship that exists between posts and
comments.

TIP: For more information on routing, see the "Rails Routing from the Outside
In":routing.html guide.

h4. Generating a Controller

With the model in hand, you can turn your attention to creating a matching
controller. Again, there's a generator for this:

<shell>
$ rails generate controller Comments
</shell>

This creates six files and one empty directory:

|_.File/Directory                             |_.Purpose                                 |
| app/controllers/comments_controller.rb      | The Comments controller                  |
| app/views/comments/                         | Views of the controller are stored here  |
| test/functional/comments_controller_test.rb | The functional tests for the controller  |
| app/helpers/comments_helper.rb              | A view helper file                       |
| test/unit/helpers/comments_helper_test.rb   | The unit tests for the helper            |
| app/assets/javascripts/comment.js.coffee    | CoffeeScript for the controller          |
| app/assets/stylesheets/comment.css.scss     | Cascading style sheet for the controller |

Like with any blog, our readers will create their comments directly after
reading the post, and once they have added their comment, will be sent back to
the post show page to see their comment now listed. Due to this, our
+CommentsController+ is there to provide a method to create comments and delete
spam comments when they arrive.

So first, we'll wire up the Post show template
(+/app/views/posts/show.html.erb+) to let us make a new comment:

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

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

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

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

<h2>Add a comment:</h2>
<%= form_for([@post, @post.comments.build]) do |f| %>
  <div class="field">
    <%= f.label :commenter %><br />
    <%= f.text_field :commenter %>
  </div>
  <div class="field">
    <%= f.label :body %><br />
    <%= f.text_area :body %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

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

This adds a form on the +Post+ show page that creates a new comment by
calling the +CommentsController+ +create+ action. Let's wire that up:

<ruby>
class CommentsController < ApplicationController
  def create
    @post = Post.find(params[:post_id])
    @comment = @post.comments.create(params[:comment])
    redirect_to post_path(@post)
  end
end
</ruby>

You'll see a bit more complexity here than you did in the controller for posts.
That's a side-effect of the nesting that you've set up. Each request for a
comment has to keep track of the post to which the comment is attached, thus the
initial call to the +find+ method of the +Post+ model to get the post in question.

In addition, the code takes advantage of some of the methods available for an
association. We use the +create+ method on +@post.comments+ to create and save
the comment. This will automatically link the comment so that it belongs to that
particular post.

Once we have made the new comment, we send the user back to the original post
using the +post_path(@post)+ helper. As we have already seen, this calls the
+show+ action of the +PostsController+ which in turn renders the +show.html.erb+
template. This is where we want the comment to show, so let's add that to the
+app/views/posts/show.html.erb+.

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

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

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

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

<h2>Comments</h2>
<% @post.comments.each do |comment| %>
  <p>
    <b>Commenter:</b>
    <%= comment.commenter %>
  </p>

  <p>
    <b>Comment:</b>
    <%= comment.body %>
  </p>
<% end %>

<h2>Add a comment:</h2>
<%= form_for([@post, @post.comments.build]) do |f| %>
  <div class="field">
    <%= f.label :commenter %><br />
    <%= f.text_field :commenter %>
  </div>
  <div class="field">
    <%= f.label :body %><br />
    <%= f.text_area :body %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

<br />

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

Now you can add posts and comments to your blog and have them show up in the
right places.

h3. Refactoring

Now that we have posts and comments working, take a look at the
+app/views/posts/show.html.erb+ template. It is getting long and awkward. We can
use partials to clean it up.

h4. Rendering Partial Collections

First we will make a comment partial to extract showing all the comments for the
post. Create the file +app/views/comments/_comment.html.erb+ and put the
following into it:

<erb>
<p>
  <b>Commenter:</b>
  <%= comment.commenter %>
</p>

<p>
  <b>Comment:</b>
  <%= comment.body %>
</p>
</erb>

Then you can change +app/views/posts/show.html.erb+ to look like the
following:

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

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

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

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

<h2>Comments</h2>
<%= render @post.comments %>

<h2>Add a comment:</h2>
<%= form_for([@post, @post.comments.build]) do |f| %>
  <div class="field">
    <%= f.label :commenter %><br />
    <%= f.text_field :commenter %>
  </div>
  <div class="field">
    <%= f.label :body %><br />
    <%= f.text_area :body %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

<br />

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

This will now render the partial in +app/views/comments/_comment.html.erb+ once
for each comment that is in the +@post.comments+ collection. As the +render+
method iterates over the <tt>@post.comments</tt> collection, it assigns each
comment to a local variable named the same as the partial, in this case
+comment+ which is then available in the partial for us to show.

h4. Rendering a Partial Form

Let us also move that new comment section out to its own partial. Again, you
create a file +app/views/comments/_form.html.erb+ containing:

<erb>
<%= form_for([@post, @post.comments.build]) do |f| %>
  <div class="field">
    <%= f.label :commenter %><br />
    <%= f.text_field :commenter %>
  </div>
  <div class="field">
    <%= f.label :body %><br />
    <%= f.text_area :body %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>
</erb>

Then you make the +app/views/posts/show.html.erb+ look like the following:

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

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

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

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

<h2>Comments</h2>
<%= render @post.comments %>

<h2>Add a comment:</h2>
<%= render "comments/form" %>

<br />

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

The second render just defines the partial template we want to render,
<tt>comments/form</tt>. Rails is smart enough to spot the forward slash in that
string and realize that you want to render the <tt>_form.html.erb</tt> file in
the <tt>app/views/comments</tt> directory.

The +@post+ object is available to any partials rendered in the view because we
defined it as an instance variable.

h3. Deleting Comments

Another important feature of a blog is being able to delete spam comments. To do
this, we need to implement a link of some sort in the view and a +DELETE+ action
in the +CommentsController+.

So first, let's add the delete link in the
+app/views/comments/_comment.html.erb+ partial:

<erb>
<p>
  <b>Commenter:</b>
  <%= comment.commenter %>
</p>

<p>
  <b>Comment:</b>
  <%= comment.body %>
</p>

<p>
  <%= link_to 'Destroy Comment', [comment.post, comment],
               :confirm => 'Are you sure?',
               :method => :delete %>
</p>
</erb>

Clicking this new "Destroy Comment" link will fire off a <tt>DELETE
/posts/:id/comments/:id</tt> to our +CommentsController+, which can then use
this to find the comment we want to delete, so let's add a destroy action to our
controller:

<ruby>
class CommentsController < ApplicationController

  def create
    @post = Post.find(params[:post_id])
    @comment = @post.comments.create(params[:comment])
    redirect_to post_path(@post)
  end

  def destroy
    @post = Post.find(params[:post_id])
    @comment = @post.comments.find(params[:id])
    @comment.destroy
    redirect_to post_path(@post)
  end

end
</ruby>

The +destroy+ action will find the post we are looking at, locate the comment
within the <tt>@post.comments</tt> collection, and then remove it from the
database and send us back to the show action for the post.


h4. Deleting Associated Objects

If you delete a post then its associated comments will also need to be deleted.
Otherwise they would simply occupy space in the database. Rails allows you to
use the +dependent+ option of an association to achieve this. Modify the Post
model, +app/models/post.rb+, as follows:

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

h3. Security

If you were to publish your blog online, anybody would be able to add, edit and
delete posts or delete comments.

Rails provides a very simple HTTP authentication system that will work nicely in
this situation.

In the +PostsController+ we need to have a way to block access to the various
actions if the person is not authenticated, here we can use the Rails
<tt>http_basic_authenticate_with</tt> method, allowing access to the requested
action if that method allows it.

To use the authentication system, we specify it at the top of our
+PostsController+, in this case, we want the user to be authenticated on every
action, except for +index+ and +show+, so we write that:

<ruby>
class PostsController < ApplicationController

  http_basic_authenticate_with :name => "dhh", :password => "secret", :except => [:index, :show]

  # GET /posts
  # GET /posts.json
  def index
    @posts = Post.all
    respond_to do |format|
# snipped for brevity
</ruby>

We also only want to allow authenticated users to delete comments, so in the
+CommentsController+ we write:

<ruby>
class CommentsController < ApplicationController

  http_basic_authenticate_with :name => "dhh", :password => "secret", :only => :destroy

  def create
    @post = Post.find(params[:post_id])
# snipped for brevity
</ruby>

Now if you try to create a new post, you will be greeted with a basic HTTP
Authentication challenge

!images/challenge.png(Basic HTTP Authentication Challenge)!

h3. Building a Multi-Model Form

Another feature of your average blog is the ability to tag posts. To implement
this feature your application needs to interact with more than one model on a
single form. Rails offers support for nested forms.

To demonstrate this, we will add support for giving each post multiple tags,
right in the form where you create the post. First, create a new model to hold
the tags:

<shell>
$ rails generate model Tag name:string post:references
</shell>

Again, run the migration to create the database table:

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

Next, edit the +post.rb+ file to create the other side of the association, and
to tell Rails (via the +accepts_nested_attributes_for+ macro) that you intend to
edit tags via posts:

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

  has_many :comments, :dependent => :destroy
  has_many :tags

  accepts_nested_attributes_for :tags, :allow_destroy => :true,
    :reject_if => proc { |attrs| attrs.all? { |k, v| v.blank? } }
end
</ruby>

The +:allow_destroy+ option tells Rails to enable destroying tags through the
nested attributes (you'll handle that by displaying a "remove" checkbox on the
view that you'll build shortly). The +:reject_if+ option prevents saving new
tags that do not have any attributes filled in.

We will modify +views/posts/_form.html.erb+ to render a partial to make a tag:

<erb>
<% @post.tags.build %>
<%= form_for(@post) do |post_form| %>
  <% 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">
    <%= post_form.label :name %><br />
    <%= post_form.text_field :name %>
  </div>
  <div class="field">
    <%= post_form.label :title %><br />
    <%= post_form.text_field :title %>
  </div>
  <div class="field">
    <%= post_form.label :content %><br />
    <%= post_form.text_area :content %>
  </div>
  <h2>Tags</h2>
  <%= render :partial => 'tags/form',
             :locals => {:form => post_form} %>
  <div class="actions">
    <%= post_form.submit %>
  </div>
<% end %>
</erb>

Note that we have changed the +f+ in +form_for(@post) do |f|+ to +post_form+ to
make it easier to understand what is going on.

This example shows another option of the render helper, being able to pass in
local variables, in this case, we want the local variable +form+ in the partial
to refer to the +post_form+ object.

We also add a <tt>@post.tags.build</tt> at the top of this form. This is to make
sure there is a new tag ready to have its name filled in by the user. If you do
not build the new tag, then the form will not appear as there is no new Tag
object ready to create.

Now create the folder <tt>app/views/tags</tt> and make a file in there called
<tt>_form.html.erb</tt> which contains the form for the tag:

<erb>
<%= form.fields_for :tags do |tag_form| %>
  <div class="field">
    <%= tag_form.label :name, 'Tag:' %>
    <%= tag_form.text_field :name %>
  </div>
  <% unless tag_form.object.nil? || tag_form.object.new_record? %>
    <div class="field">
      <%= tag_form.label :_destroy, 'Remove:' %>
      <%= tag_form.check_box :_destroy %>
    </div>
  <% end %>
<% end %>
</erb>

Finally, we will edit the <tt>app/views/posts/show.html.erb</tt> template to
show our tags.

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

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

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

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

<p>
  <b>Tags:</b>
  <%= @post.tags.map { |t| t.name }.join(", ") %>
</p>

<h2>Comments</h2>
<%= render @post.comments %>

<h2>Add a comment:</h2>
<%= render "comments/form" %>


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

With these changes in place, you'll find that you can edit a post and its tags
directly on the same view.

However, that method call <tt>@post.tags.map { |t| t.name }.join(", ")</tt> is
awkward, we could handle this by making a helper method.

h3. View Helpers

View Helpers live in <tt>app/helpers</tt> and provide small snippets of reusable
code for views. In our case, we want a method that strings a bunch of objects
together using their name attribute and joining them with a comma. As this is
for the Post show template, we put it in the PostsHelper.

Open up <tt>app/helpers/posts_helper.rb</tt> and add the following:

<erb>
module PostsHelper
  def join_tags(post)
    post.tags.map { |t| t.name }.join(", ")
  end
end
</erb>

Now you can edit the view in <tt>app/views/posts/show.html.erb</tt> to look like
this:

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

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

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

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

<p>
  <b>Tags:</b>
  <%= join_tags(@post) %>
</p>

<h2>Comments</h2>
<%= render @post.comments %>

<h2>Add a comment:</h2>
<%= render "comments/form" %>


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

h3. What's Next?

Now that you've seen your first Rails application, you should feel free to
update it and experiment on your own. But you don't have to do everything
without help. As you need assistance getting up and running with Rails, feel
free to consult these support resources:

* The "Ruby on Rails guides":index.html
* The "Ruby on Rails Tutorial":http://railstutorial.org/book
* The "Ruby on Rails mailing list":http://groups.google.com/group/rubyonrails-talk
* The "#rubyonrails":irc://irc.freenode.net/#rubyonrails channel on irc.freenode.net

Rails also comes with built-in help that you can generate using the rake command-line utility:

* Running +rake doc:guides+ will put a full copy of the Rails Guides in the +doc/guides+ folder of your application. Open +doc/guides/index.html+ in your web browser to explore the Guides.
* Running +rake doc:rails+ will put a full copy of the API documentation for Rails in the +doc/api+ folder of your application. Open +doc/api/index.html+ in your web browser to explore the API documentation.

h3. Configuration Gotchas

The easiest way to work with Rails is to store all external data as UTF-8. If
you don't, Ruby libraries and Rails will often be able to convert your native
data into UTF-8, but this doesn't always work reliably, so you're better off
ensuring that all external data is UTF-8.

If you have made a mistake in this area, the most common symptom is a black
diamond with a question mark inside appearing in the browser. Another common
symptom is characters like "ü" appearing instead of "ü". Rails takes a number
of internal steps to mitigate common causes of these problems that can be
automatically detected and corrected. However, if you have external data that is
not stored as UTF-8, it can occasionally result in these kinds of issues that
cannot be automatically detected by Rails and corrected.

Two very common sources of data that are not UTF-8:
* Your text editor: Most text editors (such as Textmate), default to saving files as
  UTF-8. If your text editor does not, this can result in special characters that you
  enter in your templates (such as é) to appear as a diamond with a question mark inside
  in the browser. This also applies to your I18N translation files.
  Most editors that do not already default to UTF-8 (such as some versions of
  Dreamweaver) offer a way to change the default to UTF-8. Do so.
* Your database. Rails defaults to converting data from your database into UTF-8 at
  the boundary. However, if your database is not using UTF-8 internally, it may not
  be able to store all characters that your users enter. For instance, if your database
  is using Latin-1 internally, and your user enters a Russian, Hebrew, or Japanese
  character, the data will be lost forever once it enters the database. If possible,
  use UTF-8 as the internal storage of your database.