rails--rails/guides/source/getting_started.md

Getting Started with Rails

This guide covers getting up and running with Ruby on Rails.

After reading this guide, you will know:

• How to install Rails, create a new Rails application, and connect 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.

---

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 language version 1.9.3 or higher
• The RubyGems packaging system
  • To learn more about RubyGems, please read the RubyGems User Guide
• A working installation of the SQLite3 Database

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
• Programming Ruby
• Why's (Poignant) Guide to Ruby

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 the "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.

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.

By following along with this guide, you'll create a Rails project called blog, 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 superuser and regular user terminal prompts respectively in a UNIX-like OS. If you are using Windows, your prompt will look something like c:\source_code>

Installing Rails

Open up a command line prompt. On Mac OS X open Terminal.app, on Windows choose "Run" from your Start menu and type 'cmd.exe'. Any commands prefaced with a dollar sign $ should be run in the command line. Verify that you have a current version of Ruby installed:

$ ruby -v
ruby 1.9.3p327

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

$ gem install rails

TIP. A number of tools exist to help you quickly install Ruby and Ruby on Rails on your system. Windows users can use Rails Installer, while Mac OS X users can use Rails One Click.

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

$ rails --version

If it says something like "Rails 3.2.9", you are ready to continue.

Creating the Blog Application

Rails comes with a number of scripts called generators that are designed to make your development life easier by creating everything that's necessary to start working on a particular task. One of these is the new application generator, which will provide you with the foundation of a fresh 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:

$ rails new blog

This will create a Rails application called Blog in a directory called blog and install the gem dependencies that are already mentioned in Gemfile using bundle install.

TIP: You can see all of the command line options that the Rails application builder accepts by running rails new -h.

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

$ cd blog

The rails new blog command we ran above created a folder in your working directory called blog. The blog directory has a number of auto-generated files and folders that make up the structure of a Rails application. Most of the work in this tutorial will happen in the app/ 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, helpers, mailers and assets for your application. You'll focus on this folder for the remainder of this guide.
bin/	Contains the rails script that starts your app and can contain other scripts you use to deploy or run your application.
config/	Configure your application's runtime rules, routes, database, and more. This is covered in more detail in Configuring Rails Applications
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.
Gemfile 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
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.
test/	Unit tests, fixtures, and other test apparatus. These are covered in Testing Rails Applications
tmp/	Temporary files (like cache, pid and session 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).

Hello, Rails!

To begin with, let's get some text up on screen quickly. To do this, you need to get your Rails application server running.

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:

$ rails server

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.

This will fire up WEBrick, a webserver built into Ruby by default. To see your application in action, open a browser window and navigate to http://localhost:3000. You should see the Rails default information page:

TIP: To stop the web server, hit Ctrl+C in the terminal window where it's running. To verify the server has stopped you should see your command prompt cursor again. For most UNIX-like systems including Mac OS X this will be a dollar sign $. In development mode, Rails does not generally require you to restart 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.

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. Routing decides which controller receives which requests. Often, there is 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, you will need to run the "controller" generator and tell it you want a controller called "welcome" with an action called "index", just like this:

$ rails generate controller welcome index

Rails will create several files and a route for you.

create  app/controllers/welcome_controller.rb
 route  get "welcome/index"
invoke  erb
create    app/views/welcome
create    app/views/welcome/index.html.erb
invoke  test_unit
create    test/controllers/welcome_controller_test.rb
invoke  helper
create    app/helpers/welcome_helper.rb
invoke    test_unit
create      test/helpers/welcome_helper_test.rb
invoke  assets
invoke    coffee
create      app/assets/javascripts/welcome.js.coffee
invoke    scss
create      app/assets/stylesheets/welcome.css.scss

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:

<h1>Hello, Rails!</h1>

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. At the moment, "Welcome Aboard" is occupying that spot.

Next, you have to tell Rails where your actual home page is located.

Open the file config/routes.rb in your editor.

Blog::Application.routes.draw do
  get "welcome/index"

  # The priority is based upon order of creation:
  # first created -> highest priority.
  # ...
  # You can have the root of your site routed with "root"
  # root to: "welcome#index"

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:

root to: "welcome#index"

The root to: "welcome#index" tells Rails to map requests to the root of the application to the welcome controller's index action and get "welcome/index" tells Rails to map requests to http://localhost:3000/welcome/index 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 in your browser, you'll see the Hello, Rails! message you put into app/views/welcome/index.html.erb, indicating that this new route is indeed going to WelcomeController's index action and is rendering the view correctly.

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

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 "C" and the "R" from CRUD: creation and reading. The form for doing this will look like this:

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

Laying down the ground work

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 — Rails will give you a routing error:

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, besides the root route you defined earlier, 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:

get "posts/new"

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 you 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 and you'll see another routing error:

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. The solution to this particular problem is simple: you need to create a controller called PostsController. You can do this by running this command:

$ rails g controller posts

If you open up the newly generated app/controllers/posts_controller.rb you'll see a fairly empty controller:

class PostsController < ApplicationController
end

A controller is simply a class that is defined to inherit from ApplicationController. It's inside this class that you'll define methods that will become the actions for this controller. These actions will perform CRUD operations on the posts within our system.

If you refresh http://localhost:3000/posts/new now, you'll get a new error:

This error indicates that Rails cannot find the new action inside the PostsController that you just generated. This is because when controllers are generated in Rails they are empty by default, unless you tell it you wanted actions during the generation process.

To manually define an action inside a controller, all you need to do is to define a new method inside the controller. Open app/controllers/posts_controller.rb and inside the PostsController class, define a new method like this:

def new
end

With the new method defined in PostsController, if you refresh http://localhost:3000/posts/new you'll see another error:

You're getting this error now because Rails expects plain actions like this one to have views associated with them to display their information. With no view available, Rails errors out.

In the above image, the bottom line has been truncated. Let's see what the full thing looks like:

Missing template posts/new, application/new with {locale:[:en], formats:[:html], handlers:[:erb, :builder, :coffee]}. Searched in: * "/path/to/blog/app/views"

That's quite a lot of text! Let's quickly go through and understand what each part of it does.

The first part identifies what template is missing. In this case, it's the posts/new template. Rails will first look for this template. If not found, then it will attempt to load a template called application/new. It looks for one here because the PostsController inherits from ApplicationController.

The next part of the message contains a hash. The :locale key in this hash simply indicates what spoken language template should be retrieved. By default, this is the English — or "en" — template. The next key, :formats specifies the format of template to be served in response. The default format is :html, and so Rails is looking for an HTML template. The final key, :handlers, is telling us what template handlers could be used to render our template. :erb is most commonly used for HTML templates, :builder is used for XML templates, and :coffee uses CoffeeScript to build JavaScript templates.

The final part of this message tells us where Rails has looked for the templates. Templates within a basic Rails application like this are kept in a single location, but in more complex applications it could be many different paths.

The simplest template that would work in this case would be one located at app/views/posts/new.html.erb. The extension of this file name is key: the first extension is the format of the template, and the second extension is the handler that will be used. Rails is attempting to find a template called posts/new within app/views for the application. The format for this template can only be html and the handler must be one of erb, builder or coffee. Because you want to create a new HTML form, you will be using the ERB language. Therefore the file should be called posts/new.html.erb and needs to be located inside the app/views directory of the application.

Go ahead now and create a new file at app/views/posts/new.html.erb and write this content in it:

<h1>New Post</h1>

When you refresh http://localhost:3000/posts/new you'll now see that the page has a title. The route, controller, action and view are now working harmoniously! It's time to create the form for a new post.

The first form

To create a form within this template, you will use a form builder. The primary form builder for Rails is provided by a helper method called form_for. To use this method, add this code into app/views/posts/new.html.erb:

<%= form_for :post do |f| %>
  <p>
    <%= f.label :title %><br>
    <%= f.text_field :title %>
  </p>

  <p>
    <%= f.label :text %><br>
    <%= f.text_area :text %>
  </p>

  <p>
    <%= f.submit %>
  </p>
<% end %>

If you refresh the page now, you'll see the exact same form as in the example. Building forms in Rails is really just that easy!

When you call form_for, you pass it an identifying object for this form. In this case, it's the symbol :post. This tells the form_for helper what this form is for. Inside the block for this method, the FormBuilder object — represented by f — is used to build two labels and two text fields, one each for the title and text of a post. Finally, a call to submit on the f object will create a submit button for the form.

There's one problem with this form though. If you inspect the HTML that is generated, by viewing the source of the page, you will see that the action attribute for the form is pointing at /posts/new. This is a problem because this route goes to the very page that you're on right at the moment, and that route should only be used to display the form for a new post.

The form needs to use a different URL in order to go somewhere else. This can be done quite simply with the :url option of form_for. Typically in Rails, the action that is used for new form submissions like this is called "create", and so the form should be pointed to that action.

Edit the form_for line inside app/views/posts/new.html.erb to look like this:

<%= form_for :post, url: { action: :create } do |f| %>

In this example, a Hash object is passed to the :url option. What Rails will do with this is that it will point the form to the create action of the current controller, the PostsController, and will send a POST request to that route. For this to work, you will need to add a route to config/routes.rb, right underneath the one for "posts/new":

post "posts" => "posts#create"

By using the post method rather than the get method, Rails will define a route that will only respond to POST methods. The POST method is the typical method used by forms all over the web.

With the form and its associated route defined, you will be able to fill in the form and then click the submit button to begin the process of creating a new post, so go ahead and do that. When you submit the form, you should see a familiar error:

You now need to create the create action within the PostsController for this to work.

Creating posts

To make the "Unknown action" go away, you can define a create action within the PostsController class in app/controllers/posts_controller.rb, underneath the new action:

class PostsController < ApplicationController
  def new
  end

  def create
  end
end

If you re-submit the form now, you'll see another familiar error: a template is missing. That's ok, we can ignore that for now. What the create action should be doing is saving our new post to a database.

When a form is submitted, the fields of the form are sent to Rails as parameters. These parameters can then be referenced inside the controller actions, typically to perform a particular task. To see what these parameters look like, change the create action to this:

def create
  render text: params[:post].inspect
end

The render method here is taking a very simple hash with a key of text and value of params[:post].inspect. The params method is the object which represents the parameters (or fields) coming in from the form. The params method returns an ActiveSupport::HashWithIndifferentAccess object, which allows you to access the keys of the hash using either strings or symbols. In this situation, the only parameters that matter are the ones from the form.

If you re-submit the form one more time you'll now no longer get the missing template error. Instead, you'll see something that looks like the following:

{"title"=>"First post!", "text"=>"This is my first post."}

This action is now displaying the parameters for the post that are coming in from the form. However, this isn't really all that helpful. Yes, you can see the parameters but nothing in particular is being done with them.

Creating the Post model

Models in Rails use a singular name, and their corresponding database tables use a plural name. Rails provides a generator for creating models, which most Rails developers tend to use when creating new models. To create the new model, run this command in your terminal:

$ rails generate model Post title:string text:text

With that command we told Rails that we want a Post model, together with a title attribute of type string, and a text attribute of type text. Those attributes are automatically added to the posts table in the database and mapped to the Post model.

Rails responded by creating a bunch of files. For now, we're only interested in app/models/post.rb and db/migrate/20120419084633_create_posts.rb (your name could be a bit different). The latter is responsible for creating the database structure, which is what we'll look at next.

TIP: Active Record is smart enough to automatically map column names to model attributes, which means you don't have to declare attributes inside Rails models, as that will be done automatically by Active Record.

Running a Migration

As we've just seen, rails generate model created a database migration file inside the db/migrate directory. 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/20120419084633_create_posts.rb file (remember, yours will have a slightly different name), here's what you'll find:

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

      t.timestamps
    end
  end
end

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 one string column and a text column. It also creates two timestamp fields to allow Rails to track post creation and update times.

TIP: For more information about migrations, refer to Rails Database Migrations.

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

$ rake db:migrate

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

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

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: rake db:migrate RAILS_ENV=production.

Saving data in the controller

Back in posts_controller, we need to change the create action to use the new Post model to save the data in the database. Open app/controllers/posts_controller.rb and change the create action to look like this:

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

  @post.save
  redirect_to action: :show, id: @post.id
end

Here's what's going on: every Rails model can be initialized with its respective attributes, which are automatically mapped to the respective database columns. In the first line we do just that (remember that params[:post] contains the attributes we're interested in). Then, @post.save is responsible for saving the model in the database. Finally, we redirect the user to the show action, which we'll define later.

TIP: As we'll see later, @post.save returns a boolean indicating whether the model was saved or not.

Showing Posts

If you submit the form again now, Rails will complain about not finding the show action. That's not very useful though, so let's add the show action before proceeding. Open config/routes.rb and add the following route:

get "posts/:id" => "posts#show"

The special syntax :id tells rails that this route expects an :id parameter, which in our case will be the id of the post. Note that this time we had to specify the actual mapping, posts#show because otherwise Rails would not know which action to render.

As we did before, we need to add the show action in app/controllers/posts_controller.rb and its respective view.

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

A couple of things to note. We use Post.find to find the post we're interested in. We also use an instance variable (prefixed by @) to hold a reference to the post object. We do this because Rails will pass all instance variables to the view.

Now, create a new file app/view/posts/show.html.erb with the following content:

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

<p>
  <strong>Text:</strong>
  <%= @post.text %>
</p>

Finally, if you now go to http://localhost:3000/posts/new you'll be able to create a post. Try it!

Listing all posts

We still need a way to list all our posts, so let's do that. As usual, we'll need a route placed into config/routes.rb:

get "posts" => "posts#index"

And an action for that route inside the PostsController in the app/controllers/posts_controller.rb file:

def index
  @posts = Post.all
end

And then finally a view for this action, located at app/views/posts/index.html.erb:

<h1>Listing posts</h1>

<table>
  <tr>
    <th>Title</th>
    <th>Text</th>
  </tr>

  <% @posts.each do |post| %>
    <tr>
      <td><%= post.title %></td>
      <td><%= post.text %></td>
    </tr>
  <% end %>
</table>

Now if you go to http://localhost:3000/posts you will see a list of all the posts that you have created.

Adding links

You can now create, show, and list posts. Now let's add some links to navigate through pages.

Open app/views/welcome/index.html.erb and modify it as follows:

<h1>Hello, Rails!</h1>
<%= link_to "My Blog", controller: "posts" %>

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.

Let's add links to the other views as well, starting with adding this "New Post" link to app/views/posts/index.html.erb, placing it above the <table> tag:

<%= link_to 'New post', action: :new %>

This link will allow you to bring up the form that lets you create a new post. You should also add a link to this template — app/views/posts/new.html.erb — to go back to the index action. Do this by adding this underneath the form in this template:

<%= form_for :post do |f| %>
  ...
<% end %>

<%= link_to 'Back', action: :index %>

Finally, add another link to the app/views/posts/show.html.erb template to go back to the index action as well, so that people who are viewing a single post can go back and view the whole list again:

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

<p>
  <strong>Text:</strong>
  <%= @post.text %>
</p>

<%= link_to 'Back', action: :index %>

TIP: If you want to link to an action in the same controller, you don't need to specify the :controller option, as Rails will use the current controller by default.

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 when a change is made.

Allowing the update of fields

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

class Post < ActiveRecord::Base
end

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.

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:

class Post < ActiveRecord::Base
  validates :title, presence: true,
                    length: { minimum: 5 }
end

These changes will ensure that all posts have a title that 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

With the validation now in place, when you call @post.save on an invalid post, it will return false. If you open app/controllers/posts_controller.rb again, you'll notice that we don't check the result of calling @post.save inside the create action. If @post.save fails in this situation, we need to show the form back to the user. To do this, change the new and create actions inside app/controllers/posts_controller.rb to these:

def new
  @post = Post.new
end

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

  if @post.save
    redirect_to action: :show, id: @post.id
  else
    render 'new'
  end
end

The new action is now creating a new instance variable called @post, and you'll see why that is in just a few moments.

Notice that inside the create action we use render instead of redirect_to when save returns false. The render method is used so that the @post object is passed back to the new template when it is rendered. This rendering is done within the same request as the form submission, whereas the redirect_to will tell the browser to issue another request.

If you reload http://localhost:3000/posts/new and try to save a post without a title, Rails will send you back to the form, but that's not very useful. You need to tell the user that something went wrong. To do that, you'll modify app/views/posts/new.html.erb to check for error messages:

<%= form_for :post, url: { action: :create } 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 %>
  <p>
    <%= f.label :title %><br>
    <%= f.text_field :title %>
  </p>

  <p>
    <%= f.label :text %><br>
    <%= f.text_area :text %>
  </p>

  <p>
    <%= f.submit %>
  </p>
<% end %>

<%= link_to 'Back', action: :index %>

A few things are going on. We check if there are any errors with @post.errors.any?, and in that case we show a list of all errors with @post.errors.full_messages.

pluralize is a rails helper that takes a number and a string as its arguments. If the number is greater than one, the string will be automatically pluralized.

The reason why we added @post = Post.new in posts_controller is that otherwise @post would be nil in our view, and calling @post.errors.any? would throw an error.

TIP: Rails automatically wraps fields that contain an error with a div with class field_with_errors. You can define a css rule to make them standout.

Now you'll get a nice error message when saving a post without title when you attempt to do just that on the new post form (http://localhost:3000/posts/new).

Updating Posts

We've covered the "CR" part of CRUD. Now let's focus on the "U" part, updating posts.

The first step we'll take is adding an edit action to posts_controller.

Start by adding a route to config/routes.rb:

get "posts/:id/edit" => "posts#edit"

And then add the controller action:

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

The view will contain a form similar to the one we used when creating new posts. Create a file called app/views/posts/edit.html.erb and make it look as follows:

<h1>Editing post</h1>

<%= form_for :post, url: { action: :update, id: @post.id },
method: :patch 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 %>
  <p>
    <%= f.label :title %><br>
    <%= f.text_field :title %>
  </p>

  <p>
    <%= f.label :text %><br>
    <%= f.text_area :text %>
  </p>

  <p>
    <%= f.submit %>
  </p>
<% end %>

<%= link_to 'Back', action: :index %>

This time we point the form to the update action, which is not defined yet but will be very soon.

The method: :patch option tells Rails that we want this form to be submitted via the PUT HTTP method which is the HTTP method you're expected to use to update resources according to the REST protocol.

TIP: By default forms built with the form_for helper are sent via POST.

Next, we need to add the update action. The file config/routes.rb will need just one more line:

patch "posts/:id" => "posts#update"

And then create the update action in app/controllers/posts_controller.rb:

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

  if @post.update(params[:post])
    redirect_to action: :show, id: @post.id
  else
    render 'edit'
  end
end

The new method, update, is used when you want to update a record that already exists, and it accepts a hash containing the attributes that you want to update. As before, if there was an error updating the post we want to show the form back to the user.

TIP: You don't need to pass all attributes to update. For example, if you'd call @post.update(title: 'A new title') Rails would only update the title attribute, leaving all other attributes untouched.

Finally, we want to show a link to the edit action in the list of all the posts, so let's add that now to app/views/posts/index.html.erb to make it appear next to the "Show" link:

<table>
  <tr>
    <th>Title</th>
    <th>Text</th>
    <th></th>
    <th></th>
  </tr>

<% @posts.each do |post| %>
  <tr>
    <td><%= post.title %></td>
    <td><%= post.text %></td>
    <td><%= link_to 'Show', action: :show, id: post.id %></td>
    <td><%= link_to 'Edit', action: :edit, id: post.id %></td>
  </tr>
<% end %>
</table>

And we'll also add one to the app/views/posts/show.html.erb template as well, so that there's also an "Edit" link on a post's page. Add this at the bottom of the template:

...

<%= link_to 'Back', action: :index %>
| <%= link_to 'Edit', action: :edit, id: @post.id %>

And here's how our app looks so far:

Using partials to clean up duplication in views

Our edit page looks very similar to the new page, in fact they both share the same code for displaying the form. Let's remove some duplication by using a view partial. By convention, partial files are prefixed by an underscore.

TIP: You can read more about partials in the Layouts and Rendering in Rails guide.

Create a new file app/views/posts/_form.html.erb with the following content:

<%= 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 %>
  <p>
    <%= f.label :title %><br>
    <%= f.text_field :title %>
  </p>

  <p>
    <%= f.label :text %><br>
    <%= f.text_area :text %>
  </p>

  <p>
    <%= f.submit %>
  </p>
<% end %>

Everything except for the form_for declaration remained the same. How form_for can figure out the right action and method attributes when building the form will be explained in just a moment. For now, let's update the app/views/posts/new.html.erb view to use this new partial, rewriting it completely:

<h1>New post</h1>

<%= render 'form' %>

<%= link_to 'Back', action: :index %>

Then do the same for the app/views/posts/edit.html.erb view:

<h1>Edit post</h1>

<%= render 'form' %>

<%= link_to 'Back', action: :index %>

Point your browser to http://localhost:3000/posts/new and try creating a new post. Everything still works. Now try editing the post and you'll receive the following error:

To understand this error, you need to understand how form_for works. When you pass an object to form_for and you don't specify a :url option, Rails will try to guess the action and method options by checking if the passed object is a new record or not. Rails follows the REST convention, so to create a new Post object it will look for a route named posts_path, and to update a Post object it will look for a route named post_path and pass the current object. Similarly, rails knows that it should create new objects via POST and update them via PUT.

If you run rake routes from the console you'll see that we already have a posts_path route, which was created automatically by Rails when we defined the route for the index action. However, we don't have a post_path yet, which is the reason why we received an error before. With your server running you can view your routes by visiting localhost:3000/rails/info/routes, or you can generate them from the command line by running rake routes:

$ rake routes

    posts GET  /posts(.:format)            posts#index
posts_new GET  /posts/new(.:format)        posts#new
          POST /posts(.:format)            posts#create
          GET  /posts/:id(.:format)        posts#show
          GET  /posts/:id/edit(.:format)   posts#edit
          PUT  /posts/:id(.:format)        posts#update
     root      /                           welcome#index

To fix this, open config/routes.rb and modify the get "posts/:id" line like this:

get "posts/:id" => "posts#show", as: :post

The :as option tells the get method that we want to make routing helpers called post_url and post_path available to our application. These are precisely the methods that the form_for needs when editing a post, and so now you'll be able to update posts again.

NOTE: The :as option is available on the post, patch, put, delete and match routing methods also.

Deleting Posts

We're now ready to cover the "D" part of CRUD, deleting posts from the database. Following the REST convention, we're going to add a route for deleting posts to config/routes.rb:

delete "posts/:id" => "posts#destroy"

The delete routing method should be used for routes that destroy resources. If this was left as a typical get route, it could be possible for people to craft malicious URLs like this:

<a href='http://example.com/posts/1/destroy'>look at this cat!</a>

We use the delete method for destroying resources, and this route is mapped to the destroy action inside app/controllers/posts_controller.rb, which doesn't exist yet, but is provided below:

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

  redirect_to action: :index
end

You can call destroy on Active Record objects when you want to delete them from the database. Note that we don't need to add a view for this action since we're redirecting to the index action.

Finally, add a 'destroy' link to your index action template (app/views/posts/index.html.erb) to wrap everything together.

<h1>Listing Posts</h1>
<table>
  <tr>
    <th>Title</th>
    <th>Text</th>
    <th></th>
    <th></th>
    <th></th>
  </tr>

<% @posts.each do |post| %>
  <tr>
    <td><%= post.title %></td>
    <td><%= post.text %></td>
    <td><%= link_to 'Show', action: :show, id: post.id %></td>
    <td><%= link_to 'Edit', action: :edit, id: post.id %></td>
    <td><%= link_to 'Destroy', { action: :destroy, id: post.id },
                    method: :delete, data: { confirm: 'Are you sure?' } %></td>
  </tr>
<% end %>
</table>

Here we're using link_to in a different way. We wrap the :action and :id attributes in a hash so that we can pass those two keys in first as one argument, and then the final two keys as another argument. The :method and :'data-confirm' options are used as HTML5 attributes so that when the link is clicked, Rails will first show a confirm dialog to the user, and then submit the link with method delete. This is done via the JavaScript file jquery_ujs which is automatically included into your application's layout (app/views/layouts/application.html.erb) when you generated the application. Without this file, the confirmation dialog box wouldn't appear.

Congratulations, you can now create, show, list, update and destroy posts. In the next section will see how Rails can aid us when creating REST applications, and how we can refactor our Blog app to take advantage of it.

Going Deeper into REST

We've now covered all the CRUD actions of a REST app. We did so by declaring separate routes with the appropriate verbs into config/routes.rb. Here's how that file looks so far:

get "posts" => "posts#index"
get "posts/new"
post "posts" => "posts#create"
get "posts/:id" => "posts#show", as: :post
get "posts/:id/edit" => "posts#edit"
patch "posts/:id" => "posts#update"
delete "posts/:id" => "posts#destroy"

That's a lot to type for covering a single resource. Fortunately, Rails provides a resources method which can be used to declare a standard REST resource. Here's how config/routes.rb looks after the cleanup:

Blog::Application.routes.draw do

  resources :posts

  root to: "welcome#index"
end

If you run rake routes, you'll see that all the routes that we declared before are still available:

$ rake routes
    posts GET    /posts(.:format)          posts#index
          POST   /posts(.:format)          posts#create
 new_post GET    /posts/new(.:format)      posts#new
edit_post GET    /posts/:id/edit(.:format) posts#edit
     post GET    /posts/:id(.:format)      posts#show
          PUT    /posts/:id(.:format)      posts#update
          DELETE /posts/:id(.:format)      posts#destroy
     root        /                         welcome#index

Also, if you go through the motions of creating, updating and deleting posts the app still works as before.

TIP: In general, Rails encourages the use of resources objects in place of declaring routes manually. It was only done in this guide as a learning exercise. For more information about routing, see Rails Routing from the Outside In.

Adding a Second Model

It's time to add a second model to the application. The second model will handle comments on posts.

Generating a Model

We're going to see the same generator that we used before when creating the Post model. This time we'll create a Comment model to hold reference of post comments. Run this command in your terminal:

$ rails generate model Comment commenter:string body:text post:references

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/models/comment_test.rb	Testing harness for the comments model
test/fixtures/comments.yml	Sample comments for use in testing

First, take a look at app/models/comment.rb:

class Comment < ActiveRecord::Base
  belongs_to :post
end

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:

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

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:

$ rake db:migrate

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:

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

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 (app/models/comment.rb) that makes each comment belong to a Post:

class Comment < ActiveRecord::Base
  belongs_to :post
end

You'll need to edit app/models/post.rb to add the other side of the association:

class Post < ActiveRecord::Base
  has_many :comments
  validates :title, presence: true,
                    length: { minimum: 5 }
  [...]
end

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 guide.

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, and edit it as follows:

resources :posts do
  resources :comments
end

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 guide.

Generating a Controller

With the model in hand, you can turn your attention to creating a matching controller. Again, we'll use the same generator we used before:

$ rails generate controller Comments

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/controllers/comments_controller_test.rb	The test for the controller
app/helpers/comments_helper.rb	A view helper file
test/helpers/comments_helper_test.rb	The test 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:

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

<p>
  <strong>Text:</strong>
  <%= @post.text %>
</p>

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

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

This adds a form on the Post show page that creates a new comment by calling the CommentsController create action. The form_for call here uses an array, which will build a nested route, such as /posts/1/comments.

Let's wire up the create in app/controllers/comments_controller.rb:

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

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.

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

<p>
  <strong>Text:</strong>
  <%= @post.text %>
</p>

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

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

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

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

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

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.

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:

<p>
  <strong>Commenter:</strong>
  <%= comment.commenter %>
</p>

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

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

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

<p>
  <strong>Text:</strong>
  <%= @post.text %>
</p>

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

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

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

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 @post.comments 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.

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:

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

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

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

<p>
  <strong>Text:</strong>
  <%= @post.text %>
</p>

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

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

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

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

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:

<p>
  <strong>Commenter:</strong>
  <%= comment.commenter %>
</p>

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

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

Clicking this new "Destroy Comment" link will fire off a DELETE /posts/:post_id/comments/:id 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 (app/controllers/comments_controller.rb):

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

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

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:

class Post < ActiveRecord::Base
  has_many :comments, dependent: :destroy
  validates :title, presence: true,
                    length: { minimum: 5 }
  [...]
end

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 http_basic_authenticate_with 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 in app/controllers/posts_controller.rb:

class PostsController < ApplicationController

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

  def index
    @posts = Post.all
  end

  # snipped for brevity

We also only want to allow authenticated users to delete comments, so in the CommentsController (app/controllers/comments_controller.rb) we write:

class CommentsController < ApplicationController

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

  def create
    @post = Post.find(params[:post_id])
    ...
  end
  # snipped for brevity

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

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
• The Ruby on Rails Tutorial
• The Ruby on Rails mailing list
• The #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.

TIP: To be able to generate the Rails Guides locally with the doc:guides rake task you need to install the RedCloth gem. Add it to your Gemfile and run bundle install and you're ready to go.

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.