2-a-demo-app.md

Chapter 2 A demo app

In this chapter, we'll develop a simple demonstration application to show off some of the power of Rails. The purpose is to get a high-level overview of Ruby on Rails programming (and web development in general) by rapidly generating an application using scaffold generators. As discussed in Box1.1, the rest of the book will take the opposite approach, developing a full application incrementally and explaining each new concept as it arises, but for a quick overview (and some instant gratification) there is no substitute for scaffolding. The resulting demo app will allow us to interact with it through its URIs, giving us insight into the structure of a Rails application, including a first example of the REST architecture favored by Rails.

As with the forthcoming sample application, the demo app will consist of users and their associated microposts (thus constituting a minimalist Twitter-style app). The functionality will be utterly under-developed, and many of the steps will seem like magic, but worry not: the full sample app will develop a similar application from the ground up starting in Chapter3, and I will provide plentiful forward-references to later material. In the mean time, have patience and a little faith--the whole point of this tutorial is to take you beyond this superficial, scaffold-driven approach to achieve a deeper understanding of Rails.

[2.1 Planning the application](a-demo-app.html#sec-

planning_the_application)

In this section, we'll outline our plans for the demo application. As in Section1.2.3, we'll start by generating the application skeleton using the rails command:

$ cd ~/rails_projects
$ rails new demo_app
$ cd demo_app

Next, we'll use a text editor to update the Gemfile needed by Bundler with the contents of [Listing2.1](a-demo-app.html#code- demo_gemfile_sqlite_version_redux).

Listing 2.1. A Gemfile for the demo app.

source 'https://rubygems.org'

gem 'rails', '3.2.8'

group :development do
  gem 'sqlite3', '1.3.5'
end


# Gems used only for assets and not required
# in production environments by default.
group :assets do
  gem 'sass-rails',   '3.2.5'
  gem 'coffee-rails', '3.2.2'

  gem 'uglifier', '1.2.3'
end

gem 'jquery-rails', '2.0.2'

group :production do
  gem 'pg', '0.12.2'
end

Note that [Listing2.1](a-demo-app.html#code- demo_gemfile_sqlite_version_redux) is identical to Listing1.9.

As in Section1.4.1, we'll install the local gems while suppressing the installation of production gems using the --without production option:

$ bundle install --without production

Finally, we'll put the demo app under version control. Recall that the rails command generates a default .gitignore file, but depending on your system you may find the augmented file from Listing1.7 to be more convenient. Then initialize a Git repository and make the first commit:

$ git init
$ git add .
$ git commit -m "Initial commit"

!create_demo_repo_new

Figure 2.1: Creating a demo app repository at GitHub.(full size)

You can also optionally create a new repository (Figure2.1 and push it up to GitHub:

$ git remote add origin git@github.com:<username>/demo_app.git
$ git push -u origin master

(As with the first app, take care not to initialize the GitHub repository with a README file.)

Now we're ready to start making the app itself. The typical first step when making a web application is to create a data model, which is a representation of the structures needed by our application. In our case, the demo app will be a microblog, with only users and short (micro)posts. Thus, we'll begin with a model for users of the app (Section2.1.1, and then we'll add a model for microposts ([Section2.1.2](a-demo-app.html#sec- modeling_demo_microposts)).

2.1.1 Modeling demo users

There are as many choices for a user data model as there are different registration forms on the web; we'll go with a distinctly minimalist approach. Users of our demo app will have a unique integer identifier called id, a publicly viewable name (of type string), and an email address (also a string) that will double as a username. A summary of the data model for users appears in [Figure2.2](a-demo-app.html#fig- demo_user_model).

!demo_user_model

Figure 2.2: The data model for users.

As we'll see starting in [Section6.1.1](modeling-users.html #sec-database_migrations), the label users in Figure2.2 corresponds to a table in a database, and the id, name, and email attributes are columns in that table.

[2.1.2 Modeling demo microposts](a-demo-app.html#sec-

modeling_demo_microposts)

The core of the micropost data model is even simpler than the one for users: a micropost has only an id and a content field for the micropost's text (of type string).1 There's an additional complication, though: we want to associate each micropost with a particular user; we'll accomplish this by recording the user_id of the owner of the post. The results are shown in Figure2.3.

!demo_micropost_model

Figure 2.3: The data model for microposts.

We'll see in [Section2.3.3](a-demo-app.html#sec- demo_user_has_many_microposts) (and more fully in Chapter10 how this user_id attribute allows us to succinctly express the notion that a user potentially has many associated microposts.

2.2 The Users resource

In this section, we'll implement the users data model in Section2.1.1, along with a web interface to that model. The combination will constitute a Users resource, which will allow us to think of users as objects that can be created, read, updated, and deleted through the web via the HTTP protocol. As promised in the introduction, our Users resource will be created by a scaffold generator program, which comes standard with each Rails project. I urge you not to look too closely at the generated code; at this stage, it will only serve to confuse you.

Rails scaffolding is generated by passing the scaffold command to the rails generate script. The argument of the scaffold command is the singular version of the resource name (in this case, User), together with optional parameters for the data model's attributes:2

$ rails generate scaffold User name:string email:string
      invoke  active_record
      create    db/migrate/20111123225336_create_users.rb
      create    app/models/user.rb
      invoke    test_unit
      create      test/unit/user_test.rb
      create      test/fixtures/users.yml
       route  resources :users
      invoke  scaffold_controller
      create    app/controllers/users_controller.rb
      invoke    erb
      create      app/views/users
      create      app/views/users/index.html.erb
      create      app/views/users/edit.html.erb
      create      app/views/users/show.html.erb
      create      app/views/users/new.html.erb
      create      app/views/users/_form.html.erb
      invoke    test_unit
      create      test/functional/users_controller_test.rb
      invoke    helper
      create      app/helpers/users_helper.rb
      invoke      test_unit
      create        test/unit/helpers/users_helper_test.rb
      invoke  assets
      invoke    coffee
      create      app/assets/javascripts/users.js.coffee
      invoke    scss
      create      app/assets/stylesheets/users.css.scss
      invoke  scss
      create    app/assets/stylesheets/scaffolds.css.scss

By including name:string and email:string, we have arranged for the User model to have the form shown in [Figure2.2](a-demo-app.html #fig-demo_user_model). (Note that there is no need to include a parameter forid; it is created automatically by Rails for use as the primary key in the database.)

To proceed with the demo application, we first need to migrate the database using Rake (Box2.1:

$ bundle exec rake db:migrate
==  CreateUsers: migrating ====================================================
-- create_table(:users)
   -> 0.0017s
==  CreateUsers: migrated (0.0018s) ===========================================

This simply updates the database with our new users data model. (We'll learn more about database migrations starting in Section6.1.1 .) Note that, in order to ensure that the command uses the version of Rake corresponding to our Gemfile, we need to run rake using bundle exec.

With that, we can run the local web server using rails s, which is a shortcut for rails server:

$ rails s

Now the demo application should be ready to go at http://localhost:3000/.

Box 2.1.Rake

In the Unix tradition, the make utility has played an important role in building executable programs from source code; many a computer hacker has committed to muscle memory the line

  $ ./configure && make && sudo make install

commonly used to compile code on Unix systems (including Linux and Mac OSX).

Rake is Ruby make, a make-like language written in Ruby. Rails uses Rake extensively, especially for the innumerable little administrative tasks necessary when developing database-backed web applications. The rake db:migrate command is probably the most common, but there are many others; you can see a list of database tasks using -T db:

$ bundle exec rake -T db

To see all the Rake tasks available, run

$ bundle exec rake -T

The list is likely to be overwhelming, but don't worry, you don't have to know all (or even most) of these commands. By the end of the Rails Tutorial, you'll know all the most important ones.

2.2.1 A user tour

Visiting the root urlhttp://localhost:3000/ shows the same default Rails page shown in Figure1.3, but in generating the Users resource scaffolding we have also created a large number of pages for manipulating users. For example, the page for listing all users is at /users, and the page for making a new user is at /users/new. The rest of this section is dedicated to taking a whirlwind tour through these user pages. As we proceed, it may help to refer to [Table2.1](a-demo- app.html#table-user_urls), which shows the correspondence between pages and URIs.

URIActionPurpose

/users

index

page to list all users

/users/1

show

page to show user with id 1

/users/new

new

page to make a new user

/users/1/edit

edit

page to edit user with id 1

Table 2.1: The correspondence between pages and URIs for the Users resource.

We start with the page to show all the users in our application, called index; as you might expect, initially there are no users at all ([Figure2.4](a-demo-app.html#fig- demo_blank_user_index_rails_3)).


Figure 2.4: The initial index page for the Users resource (/users](http: //railstutorial.org/images/figures/demo_blank_user_index_rails_3-full.png)

To make a new user, we visit the new page, as shown in [Figure2.5](a-demo-app.html#fig- demo_new_user_rails_3). (Since the http://localhost:3000 part of the address is implicit whenever we are developing locally, I'll usually omit it from now on.) In Chapter7, this will become the user signup page.

!demo_new_user_rails_3

Figure 2.5: The new user page (/users/new.(full size)

We can create a user by entering name and email values in the text fields and then clicking the Create User button. The result is the user show page, as seen in Figure2.6. (The green welcome message is accomplished using the flash, which we'll learn about in Section7.4.2 Note that the URI is /users/1; as you might suspect, the number1 is simply the user'sid attribute from [Figure2.2](a -demo-app.html#fig-demo_user_model). In [Section7.1](sign- up.html#sec-showing_users), this page will become the user's profile.

!demo_show_user_rails_3

Figure 2.6: The page to show a user (/users/1](h ttp://railstutorial.org/images/figures/demo_show_user_rails_3-full.png)

To change a user's information, we visit the edit page (Figure2.7. By modifying the user information and clicking the Update User button, we arrange to change the information for the user in the demo application ([Figure2.8](a-demo-app.html#fig- demo_update_user_rails_3)). (As we'll see in detail starting in Chapter6, this user data is stored in a database back-end.) We'll add user edit/update functionality to the sample application in [Section9.1](updating-showing- and-deleting-users.html#sec-updating_users).

!demo_edit_user_rails_3

Figure 2.7: The user edit page ([/users/1/edit](http://localhost:3000/users/1/ edit)).[(full size)](http://railstutorial.org/images/figure s/demo_edit_user_rails_3-full.png)

!demo_update_user_rails_3

Figure 2.8: A user with updated information.[(full size)](h ttp://railstutorial.org/images/figures/demo_update_user_rails_3-full.png)

Now we'll create a second user by revisiting the new page and submitting a second set of user information; the resulting user index is shown in [Figure2.9](a-demo-app.html#fig- demo_user_index_two_rails_3). [Section7.1](sign-up.html #sec-showing_users) will develop the user index into a more polished page for showing all users.

!demo_user_index_two_rails_3

Figure 2.9: The user index page (/users with a second user.[(full size)](http://railstutorial.org/images/f igures/demo_user_index_two_rails_3-full.png)

Having shown how to create, show, and edit users, we come finally to destroying them ([Figure2.10](a-demo-app.html#fig- demo_destroy_user_rails_3)). You should verify that clicking on the link in Figure2.10 destroys the second user, yielding an index page with only one user. (If it doesn't work, be sure that JavaScript is enabled in your browser; Rails uses JavaScript to issue the request needed to destroy a user.) [Section9.4](updating-showing-and-deleting-users.html#sec- destroying_users) adds user deletion to the sample app, taking care to restrict its use to a special class of administrative users.

!demo_destroy_user_rails_3

Figure 2.10: Destroying a user.[(full size)](http://railstu torial.org/images/figures/demo_destroy_user_rails_3-full.png)

2.2.2 MVC in action

Now that we've completed a quick overview of the Users resource, let's examine one particular part of it in the context of the Model-View-Controller (MVC) pattern introduced in [Section1.2.6](beginning.html#sec- mvc). Our strategy will be to describe the results of a typical browser hit--a visit to the user index page at /users--in terms of MVC ([Figure2.11](a-demo-app.html#fig- mvc_detailed)).

!mvc_detailed

Figure 2.11: A detailed diagram of MVC in Rails.(full size)

1. The browser issues a request for the /users URI.
2. Rails routes /users to the index action in the Users controller.
3. The index action asks the User model to retrieve all users (User.all).
4. The User model pulls all the users from the database.
5. The User model returns the list of users to the controller.
6. The controller captures the users in the @users variable, which is passed to the index view.
7. The view uses embedded Ruby to render the page as HTML.
8. The controller passes the HTML back to the browser.3

We start with a request issued from the browser--i.e., the result of typing a URI in the address bar or clicking on a link (Step1 in Figure2.11. This request hits the Rails router (Step2), which dispatches to the proper controller action based on the URI (and, as we'll see in Box3.2, the type of request). The code to create the mapping of user URIs to controller actions for the Users resource appears in [Listing2.2](a-demo- app.html#code-rails_routes); this code effectively sets up the table of URI/action pairs seen in [Table2.1](a-demo-app.html#table- user_urls). (The strange notation :users is a symbol, which we'll learn about in [Section4.3.3](rails-flavored-ruby.html#sec- hashes_and_symbols).)

Listing 2.2. The Rails routes, with a rule for the Users resource.

config/routes.rb

DemoApp::Application.routes.draw do
  resources :users
  .
  .
  .
end

The pages from the tour in [Section2.2.1](a-demo-app.html #sec-a_user_tour) correspond to actions in the Users controller, which is a collection of related actions; the controller generated by the scaffolding is shown schematically in [Listing2.3](a-demo-app.html #code-demo_users_controller). Note the notation class UsersController < ApplicationController; this is an example of a Ruby class with inheritance. (We'll discuss inheritance briefly in Section2.3.4 and cover both subjects in more detail in Section4.4 .)

Listing 2.3. The Users controller in schematic form.

app/controllers/users_controller.rb

class UsersController < ApplicationController

  def index
    .
    .
    .
  end

  def show
    .
    .
    .
  end

  def new
    .
    .
    .
  end

  def create
    .
    .
    .
  end

  def edit
    .
    .
    .
  end

  def update
    .
    .
    .
  end

  def destroy
    .
    .
    .
  end
end

You may notice that there are more actions than there are pages; the index, show, new, and edit actions all correspond to pages from Section2.2.1, but there are additional create, update, and destroy actions as well. These actions don't typically render pages (although they sometimes do); instead, their main purpose is to modify information about users in the database. This full suite of controller actions, summarized in Table2.2, represents the implementation of the REST architecture in Rails (Box2.2, which is based on the ideas of representational state transfer identified and named by computer scientist Roy Fielding.4 Note from [Table2.2](a-demo-app.html#table- demo_RESTful_users) that there is some overlap in the URIs; for example, both the user show action and the update action correspond to the URI /users/1. The difference between them is the HTTP request method they respond to. We'll learn more about HTTP request methods starting in Section3.2.1.

HTTP requestURIAction****Purpose

GET

/users

index

page to list all users

GET

/users/1

show

page to show user with id 1

GET

/users/new

new

page to make a new user

POST

/users

create

create a new user

GET

/users/1/edit

edit

page to edit user with id 1

PUT

/users/1

update

update user with id 1

DELETE

/users/1

destroy

delete user with id 1

Table 2.2: RESTful routes provided by the Users resource in Listing2.2.

Box 2.2.REpresentational State Transfer (REST)

If you read much about Ruby on Rails web development, you'll see a lot of references to "REST", which is an acronym for REpresentational State Transfer. REST is an architectural style for developing distributed, networked systems and software applications such as the World Wide Web and web applications. Although REST theory is rather abstract, in the context of Rails applications REST means that most application components (such as users and microposts) are modeled as resources that can be created, read, updated, and deleted-- operations that correspond both to the CRUD operations of relational databases and the four fundamental HTTP request methods: POST, GET, PUT, and DELETE. (We'll learn more about HTTP requests in Section3.2.1 and especially Box3.2

As a Rails application developer, the RESTful style of development helps you make choices about which controllers and actions to write: you simply structure the application using resources that get created, read, updated, and deleted. In the case of users and microposts, this process is straightforward, since they are naturally resources in their own right. In Chapter11, we'll see an example where REST principles allow us to model a subtler problem, "following users", in a natural and convenient way.

To examine the relationship between the Users controller and the User model, let's focus on a simplified version of the index action, shown in Listing2.4. (The scaffold code is ugly and confusing, so I've suppressed it.)

Listing 2.4. The simplified user index action for the demo application.

app/controllers/users_controller.rb

class UsersController < ApplicationController

  def index
    @users = User.all
  end
  .
  .
  .
end

This index action has the line @users = User.all (Step3), which asks the User model to retrieve a list of all the users from the database (Step4), and then places them in the variable @users (pronounced "at-users") (Step5). The User model itself appears in Listing2.5; although it is rather plain, it comes equipped with a large amount of functionality because of inheritance ([Section2.3.4](a -demo-app.html#sec-inheritance_hierarchies) and Section4.4 ). In particular, by using the Rails library called Active Record, the code in Listing2.5 arranges for User.all to return all the users. (We'll learn about the attr_accessible line in [Section6.1.2.2](modeling- users.html#sec-accessible_attributes). Note: This line will not appear if you are using Rails3.2.2 or earlier.)

Listing 2.5. The User model for the demo application.

app/models/user.rb

class User < ActiveRecord::Base
  attr_accessible :email, :name
end

Once the @users variable is defined, the controller calls the view (Step6), shown in [Listing2.6](a-demo- app.html#code-demo_index_view). Variables that start with the @sign, called instance variables, are automatically available in the view; in this case, the index.html.erb view in Listing2.6 iterates through the @users list and outputs a line of HTML for each one. (Remember, you aren't supposed to understand this code right now. It is shown only for purposes of illustration.)

Listing 2.6. The view for the user index.

app/views/users/index.html.erb

<h1>Listing users</h1>

<table>
  <tr>
    <th>Name</th>
    <th>Email</th>
    <th></th>
    <th></th>
    <th></th>
  </tr>

<% @users.each do |user| %>
  <tr>
    <td><%= user.name %></td>
    <td><%= user.email %></td>
    <td><%= link_to 'Show', user %></td>
    <td><%= link_to 'Edit', edit_user_path(user) %></td>
    <td><%= link_to 'Destroy', user, method: :delete,
                                     data: { confirm: 'Are you sure?' } %></td>
  </tr>
<% end %>
</table>

<br />

<%= link_to 'New User', new_user_path %>

The view converts its contents to HTML (Step7), which is then returned by the controller to the browser for display (Step8).

[2.2.3 Weaknesses of this Users resource](a-demo-app.html#sec-

weaknesses_of_this_users_resource)

Though good for getting a general overview of Rails, the scaffold Users resource suffers from a number of severe weaknesses.

• No data validations. Our User model accepts data such as blank names and invalid email addresses without complaint.
• No authentication. We have no notion signing in or out, and no way to prevent any user from performing any operation.
• No tests. This isn't technically true--the scaffolding includes rudimentary tests--but the generated tests are ugly and inflexible, and they don't test for data validation, authentication, or any other custom requirements.
• No layout. There is no consistent site styling or navigation.
• No real understanding. If you understand the scaffold code, you probably shouldn't be reading this book.

2.3 The Microposts resource

Having generated and explored the Users resource, we turn now to the associated Microposts resource. Throughout this section, I recommend comparing the elements of the Microposts resource with the analogous user elements from Section2.2; you should see that the two resources parallel each other in many ways. The RESTful structure of Rails applications is best absorbed by this sort of repetition of form; indeed, seeing the parallel structure of Users and Microposts even at this early stage is one of the prime motivations for this chapter. (As we'll see, writing applications more robust than the toy example in this chapter takes considerable effort--we won't see the Microposts resource again until [Chapter10](user- microposts.html#top)--and I didn't want to defer its first appearance quite that far.)

2.3.1 A micropost microtour

As with the Users resource, we'll generate scaffold code for the Microposts resource using rails generate scaffold, in this case implementing the data model from [Figure2.3](a-demo-app.html#fig- demo_micropost_model):5

$ rails generate scaffold Micropost content:string user_id:integer
      invoke  active_record
      create    db/migrate/20111123225811_create_microposts.rb
      create    app/models/micropost.rb
      invoke    test_unit
      create      test/unit/micropost_test.rb
      create      test/fixtures/microposts.yml
       route  resources :microposts
      invoke  scaffold_controller
      create    app/controllers/microposts_controller.rb
      invoke    erb
      create      app/views/microposts
      create      app/views/microposts/index.html.erb
      create      app/views/microposts/edit.html.erb
      create      app/views/microposts/show.html.erb
      create      app/views/microposts/new.html.erb
      create      app/views/microposts/_form.html.erb
      invoke    test_unit
      create      test/functional/microposts_controller_test.rb
      invoke    helper
      create      app/helpers/microposts_helper.rb
      invoke      test_unit
      create        test/unit/helpers/microposts_helper_test.rb
      invoke  assets
      invoke    coffee
      create      app/assets/javascripts/microposts.js.coffee
      invoke    scss
      create      app/assets/stylesheets/microposts.css.scss
      invoke  scss
   identical    app/assets/stylesheets/scaffolds.css.scss

To update our database with the new data model, we need to run a migration as in Section2.2:

$ bundle exec rake db:migrate
==  CreateMicroposts: migrating ===============================================
-- create_table(:microposts)
   -> 0.0023s
==  CreateMicroposts: migrated (0.0026s) ======================================

Now we are in a position to create microposts in the same way we created users in Section2.2.1. As you might guess, the scaffold generator has updated the Rails routes file with a rule for Microposts resource, as seen in [Listing2.7](a -demo-app.html#code-demo_microposts_resource).6 As with users, the resources :microposts routing rule maps micropost URIs to actions in the Microposts controller, as seen in [Table2.3](a-demo-app.html#table- demo_RESTful_microposts).

Listing 2.7. The Rails routes, with a new rule for Microposts resources.

config/routes.rb

DemoApp::Application.routes.draw do
  resources :microposts
  resources :users
  .
  .
  .
end

HTTP requestURIAction****Purpose

GET

/microposts

index

page to list all microposts

GET

/microposts/1

show

page to show micropost with id 1

GET

/microposts/new

new

page to make a new micropost

POST

/microposts

create

create a new micropost

GET

/microposts/1/edit

edit

page to edit micropost with id 1

PUT

/microposts/1

update

update micropost with id 1

DELETE

/microposts/1

destroy

delete micropost with id 1

Table 2.3: RESTful routes provided by the Microposts resource in [Listing2.7](a-demo-app.html#code- demo_microposts_resource).

The Microposts controller itself appears in schematic form [Listing2.8](a-demo-app.html#code- demo_microposts_controller). Note that, apart from having MicropostsController in place of UsersController, [Listing2.8](a-demo-app.html#code- demo_microposts_controller) is identical to the code in Listing2.3. This is a reflection of the REST architecture common to both resources.

Listing 2.8. The Microposts controller in schematic form.

app/controllers/microposts_controller.rb

class MicropostsController < ApplicationController

  def index
    .
    .
    .
  end

  def show
    .
    .
    .
  end

  def new
    .
    .
    .
  end

  def create
    .
    .
    .
  end

  def edit
    .
    .
    .
  end

  def update
    .
    .
    .
  end

  def destroy
    .
    .
    .
  end
end

To make some actual microposts, we enter information at the new microposts page, /microposts/new, as seen in [Figure2.12](a-demo-app.html#fig- demo_new_micropost_rails_3).

!demo_new_micropost_rails_3

Figure 2.12: The new micropost page ([/microposts/new](http://localhost:3000/m icroposts/new)).(full size)

At this point, go ahead and create a micropost or two, taking care to make sure that at least one has a user_id of1 to match the id of the first user created in [Section2.2.1](a-demo- app.html#sec-a_user_tour). The result should look something like [Figure2.13](a-demo-app.html#fig- demo_micropost_index_rails_3).


Figure 2.13: The micropost index page (/microposts.[(full si ze)](http://railstutorial.org/images/figures/demo_micropost_index_rails_3-full .png)

[2.3.2 Putting the micro in microposts](a-demo-app.html#sec-

putting_the_micro_in_microposts)

Any _micro_post worthy of the name should have some means of enforcing the length of the post. Implementing this constraint in Rails is easy with validations; to accept microposts with at most 140 characters (a la Twitter), we use a length validation. At this point, you should open the file app/models/micropost.rb in your text editor or IDE and fill it with the contents of [Listing2.9](a-demo-app.html#code- demo_length_validation). (The use of validates in Listing2.9 is characteristic of Rails3; if you've previously worked with Rails2.3, you should compare this to the use of validates_length_of.)

Listing 2.9. Constraining microposts to be at most 140 characters.

app/models/micropost.rb

class Micropost < ActiveRecord::Base
  attr_accessible :content, :user_id
  validates :content, :length => { :maximum => 140 }
end

The code in [Listing2.9](a-demo-app.html#code- demo_length_validation) may look rather mysterious--we'll cover validations more thoroughly starting in [Section6.2](modeling- users.html#sec-user_validations)--but its effects are readily apparent if we go to the new micropost page and enter more than 140 characters for the content of the post. As seen in [Figure2.14](a-demo- app.html#fig-micropost_length_error_rails_3), Rails renders error messages indicating that the micropost's content is too long. (We'll learn more about error messages in [Section7.3.2](sign-up.html#sec- signup_error_messages).)


Figure 2.14: Error messages for a failed micropost creation.[(full size)](http://railstutorial.org/images/figu res/micropost_length_error_rails_3-full.png)

[2.3.3 A user has_many microposts](a-demo-app.html#sec-

demo_user_has_many_microposts)

One of the most powerful features of Rails is the ability to form associations between different data models. In the case of our User model, each user potentially has many microposts. We can express this in code by updating the User and Micropost models as in [Listing2.10](a-demo-app.html#code- demo_user_has_many_microposts) and [Listing2.11](a-demo- app.html#code-demo_micropost_belongs_to_user).

Listing 2.10. A user has many microposts.

app/models/user.rb

class User < ActiveRecord::Base
  attr_accessible :email, :name
  has_many :microposts
end

Listing 2.11. A micropost belongs to a user.

app/models/micropost.rb

class Micropost < ActiveRecord::Base
  attr_accessible :content, :user_id

  belongs_to :user

  validates :content, :length => { :maximum => 140 }
end

We can visualize the result of this association in [Figure2.15](a-demo-app.html#fig- micropost_user_association). Because of the user_id column in the microposts table, Rails (using Active Record) can infer the microposts associated with each user.

!micropost_user_association

Figure 2.15: The association between microposts and users.

In Chapter10 and Chapter11, we will use the association of users and microposts both to display all a user's microposts and to construct a Twitter-like micropost feed. For now, we can examine the implications of the user-micropost association by using the console, which is a useful tool for interacting with Rails applications. We first invoke the console with rails console at the command line, and then retrieve the first user from the database using User.first (putting the results in the variable first_user):7

$ rails console
>> first_user = User.first
=> #<User id: 1, name: "Michael Hartl", email: "michael@example.org",
created_at: "2011-11-03 02:01:31", updated_at: "2011-11-03 02:01:31">
>> first_user.microposts
=> [#<Micropost id: 1, content: "First micropost!", user_id: 1, created_at:
"2011-11-03 02:37:37", updated_at: "2011-11-03 02:37:37">, #<Micropost id: 2,
content: "Second micropost", user_id: 1, created_at: "2011-11-03 02:38:54",
updated_at: "2011-11-03 02:38:54">]
>> exit

(I include the last line just to demonstrate how to exit the console, and on most systems you can Ctrl-d for the same purpose.) Here we have accessed the user's microposts using the code first_user.microposts: with this code, Active Record automatically returns all the microposts with user_id equal to the id of first_user (in this case,1). We'll learn much more about the association facilities in Active Record in Chapter10 and Chapter11.

[2.3.4 Inheritance hierarchies](a-demo-app.html#sec-

inheritance_hierarchies)

We end our discussion of the demo application with a brief description of the controller and model class hierarchies in Rails. This discussion will only make much sense if you have some experience with object-oriented programming (OOP); if you haven't studied OOP, feel free to skip this section. In particular, if you are unfamiliar with classes (discussed in Section4.4, I suggest looping back to this section at a later time.

We start with the inheritance structure for models. Comparing Listing2.12 and Listing2.13, we see that both the User model and the Micropost model inherit (via the left angle bracket<) from ActiveRecord::Base, which is the base class for models provided by ActiveRecord; a diagram summarizing this relationship appears in [Figure2.16](a-demo-app.html#fig- demo_model_inheritance). It is by inheriting from ActiveRecord::Base that our model objects gain the ability to communicate with the database, treat the database columns as Ruby attributes, and soon.

Listing 2.12. The User class, with inheritance.

app/models/user.rb

class User < ActiveRecord::Base
  .
  .
  .
end

Listing 2.13. The Micropost class, with inheritance.

app/models/micropost.rb

class Micropost < ActiveRecord::Base
  .
  .
  .
end

!demo_model_inheritance

Figure 2.16: The inheritance hierarchy for the User and Micropost models.

The inheritance structure for controllers is only slightly more complicated. Comparing [Listing2.14](a-demo-app.html#code- demo_users_controller_class) and [Listing2.15](a-demo- app.html#code-demo_microposts_controller_class), we see that both the Users controller and the Microposts controller inherit from the Application controller. Examining [Listing2.16](a-demo-app.html#code- demo_application_controller_class), we see that ApplicationController itself inherits from ActionController::Base; this is the base class for controllers provided by the Rails library Action Pack. The relationships between these classes is illustrated in [Figure2.17](a-demo-app.html#fig- demo_controller_inheritance).

Listing 2.14. The UsersController class, with inheritance.

app/controllers/users_controller.rb

class UsersController < ApplicationController
  .
  .
  .
end

Listing 2.15. The MicropostsController class, with inheritance.

app/controllers/microposts_controller.rb

class MicropostsController < ApplicationController
  .
  .
  .
end

Listing 2.16. The ApplicationController class, with inheritance.

app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  .
  .
  .
end

!demo_controller_inheritance

Figure 2.17: The inheritance hierarchy for the Users and Microposts controllers.

As with model inheritance, by inheriting ultimately from ActionController::Base both the Users and Microposts controllers gain a large amount of functionality, such as the ability to manipulate model objects, filter inbound HTTP requests, and render views as HTML. Since all Rails controllers inherit from ApplicationController, rules defined in the Application controller automatically apply to every action in the application. For example, in [Section8.2.1](sign-in-sign-out.html#sec- remember_me) we'll see how to include helpers for signing in and signing out of all of the sample application's controllers.

2.3.5 Deploying the demo app

With the completion of the Microposts resource, now is a good time to push the repository up to GitHub:

$ git add .
$ git commit -m "Finish demo app"
$ git push

Ordinarily, you should make smaller, more frequent commits, but for the purposes of this chapter a single big commit at the end is fine.

At this point, you can also deploy the demo app to Heroku as in Section1.4:

$ heroku create --stack cedar
$ git push heroku master

Finally, migrate the production database (see below if you get a deprecation warning):

$ heroku run rake db:migrate

This updates the database at Heroku with the necessary user/micropost data model. You may get a deprecation warning regarding assets in vendor/plugins, which you should ignore since there aren't any plugins in that directory.

2.4 Conclusion

We've come now to the end of the 30,000-foot view of a Rails application. The demo app developed in this chapter has several strengths and a host of weaknesses.

Strengths

• High-level overview of Rails
• Introduction to MVC
• First taste of the REST architecture
• Beginning data modeling
• A live, database-backed web application in production

Weaknesses

• No custom layout or styling
• No static pages (like "Home" or "About")
• No user passwords
• No user images
• No signing in
• No security
• No automatic user/micropost association
• No notion of "following" or "followed"
• No micropost feed
• No test-driven development
• No real understanding

The rest of this tutorial is dedicated to building on the strengths and eliminating the weaknesses.

«Chapter 1 From zero to deploy Chapter 3 Mostly static pages»

1. When modeling longer posts, such as those for a normal (non-micro) blog, you should use the text type in place of string.↑
2. The name of the scaffold follows the convention of models, which are singular, rather than resources and controllers, which are plural. Thus, we have User instead Users.↑
3. Some references indicate that the view returns the HTML directly to the browser (via a web server such as Apache or Nginx). Regardless of the implementation details, I prefer to think of the controller as a central hub through which all the application's information flows.↑
4. Fielding, Roy Thomas. Architectural Styles and the Design of Network-based Software Architectures. Doctoral dissertation, University of California, Irvine, 2000.↑
5. As with the User scaffold, the scaffold generator for microposts follows the singular convention of Rails models; thus, we have generate Micropost.↑
6. The scaffold code may have extra newlines compared to Listing2.7. This is not a cause for concern, as Ruby ignores extra newlines.↑
7. Your console prompt might be something like ruby-1.9.3-head >, but the examples use>> since Ruby versions will vary.↑