Chapter 2 A toy app

2.1.1 A toy model for users

There are as many choices for a user data model as there are different registration forms on the web; for simplicity, we’ll go with a distinctly minimalist approach. Users of our toy app will have a unique identifier called id (of type integer), a publicly viewable name (of type string), and an email address (also of type string) that will double as a unique username. A summary of the data model for users appears in Figure 2.2.

Figure 2.2: The data model for users.

As we’ll see starting in Section 6.1.1, the label users in Figure 2.2 corresponds to a table in a database, and the id, name, and email attributes are columns in that table.

2.1.2 A toy model for 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 text).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 Figure 2.3.

Figure 2.3: The data model for microposts.

We’ll see in Section 2.3.3 (and more fully in Chapter 13) how this user_id attribute allows us to succinctly express the notion that a user potentially has many associated microposts.

2.2.1 A user tour

If we visit the root URL at / (read “slash”, as noted in Section 1.3.4), we get the same “hello, world!” page shown in Figure 1.14, 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 Table 2.1, which shows the correspondence between pages and URLs.

We start with the page to show all the users in our application, called index and located at /users. As you might expect, initially there are no users at all (Figure 2.4).

Figure 2.4: The initial index page for the Users resource (/users).

To make a new user, we visit the new page at /users/new, as shown in Figure 2.5. In Chapter 7, this will become the user signup page.

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

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 at /users/1, as seen in Figure 2.6. (The green welcome message is accomplished using the flash, which we’ll learn about in Section 7.4.2.) Note that the URL is /users/1; as you might suspect, the number 1 is simply the user’s id attribute from Figure 2.2. In Section 7.1, this page will become the user’s profile page.

Figure 2.6: The page to show a user (/users/1).

To change a user’s information, we visit the edit page at /users/1/edit (Figure 2.7). By modifying the user information and clicking the Update User button, we arrange to change the information for the user in the toy application (Figure 2.8). (As we’ll see in detail starting in Chapter 6, this user data is stored in a database back-end.) We’ll add user edit/update functionality to the sample application in Section 10.1.

Figure 2.7: The user edit page (/users/1/edit).

Figure 2.8: A user with updated information.

Now we’ll create a second user by revisiting the new page at /users/new and submitting a second set of user information. The resulting user index is shown in Figure 2.9. Section 7.1 will develop the user index into a more polished page for showing all users.

Figure 2.9: The user index page (/users) with a second user.

Having shown how to create, show, and edit users, we come finally to destroying them (Figure 2.10). You should verify that clicking on the link in Figure 2.10 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.) Section 10.4 adds user deletion to the sample app, taking care to restrict its use to a special class of administrative users.

Figure 2.10: Destroying a user.

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 Section 1.3.3. 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 (Figure 2.11).

Figure 2.11: A detailed diagram of MVC in Rails.

Here is a summary of the steps shown in Figure 2.11:

1. The browser issues a request for the /users URL.
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

Now let’s take a look at the above steps in more detail. We start with a request issued from the browser—i.e., the result of typing a URL in the address bar or clicking on a link (Step 1 in Figure 2.11). This request hits the Rails router (Step 2), which dispatches the request to the proper controller action based on the URL (and, as we’ll see in Box 3.2, the type of request). The code to create the mapping of user URLs to controller actions for the Users resource appears in Listing 2.5. This code effectively sets up the table of URL/action pairs seen in Table 2.1. (The strange notation :users is a symbol, which we’ll learn about in Section 4.3.3.)

Rails.application.routes.draw do
  resources :users
  root 'application#hello'
end

While we’re looking at the routes file, let’s take a moment to associate the root route with the users index, so that “slash” goes to /users. Recall from Listing 2.3 that we added the root route

root 'application#hello'

so that the root route went to the hello action in the Application controller. In the present case, we want to use the index action in the Users controller, which we can arrange using the code shown in Listing 2.6.

Rails.application.routes.draw do
  resources :users
  root 'users#index'
end

A controller contains a collection of related actions, and the pages from the tour in Section 2.2.1 correspond to actions in the Users controller. The controller generated by the scaffolding is shown schematically in Listing 2.7. Note the code class UsersController < ApplicationController, which is an example of a Ruby class with inheritance. (We’ll discuss inheritance briefly in Section 2.3.4 and cover both subjects in more detail in Section 4.4.)

class UsersController < ApplicationController
  .
  .
  .
  def index
    .
    .
    .
  end

  def show
    .
    .
    .
  end

  def new
    .
    .
    .
  end

  def edit
    .
    .
    .
  end

  def create
    .
    .
    .
  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 Section 2.2.1, but there are additional create, update, and destroy actions as well. These actions don’t typically render pages (although they can); instead, their main purpose is to modify information about users in the database. This full suite of controller actions, summarized in Table 2.2, represents the implementation of the REST architecture in Rails (Box 2.2), which is based on the ideas of representational state transfer identified and named by computer scientist Roy Fielding.⁴ Note from Table 2.2 that there is some overlap in the URLs; for example, both the user show action and the update action correspond to the URL /users/1. The difference between them is the HTTP request method they respond to. We’ll learn more about HTTP request methods starting in Section 3.3.

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 Listing 2.8. (Learning how to read code even when you don’t fully understand it is an important aspect of technical sophistication (Box 1.1).)

class UsersController < ApplicationController
  .
  .
  .
  def index
    @users = User.all
  end
  .
  .
  .
end

This index action has the line @users = User.all (Step 3 in Figure 2.11), which asks the User model to retrieve a list of all the users from the database (Step 4), and then places them in the variable @users (pronounced “at-users”) (Step 5). The User model itself appears in Listing 2.9; although it is rather plain, it comes equipped with a large amount of functionality because of inheritance (Section 2.3.4 and Section 4.4). In particular, by using the Rails library called Active Record, the code in Listing 2.9 arranges for User.all to return all the users in the database.

class User < ApplicationRecord
end

Once the @users variable is defined, the controller calls the view (Step 6), shown in Listing 2.10. Variables that start with the @ sign, called instance variables, are automatically available in the views; in this case, the index.html.erb view in Listing 2.10 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.)

<h1>Listing users</h1>

<table>
  <thead>
    <tr>
      <th>Name</th>
      <th>Email</th>
      <th colspan="3"></th>
    </tr>
  </thead>

<% @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 (Step 7), which is then returned by the controller to the browser for display (Step 8).

2.2.3 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 of logging 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 don’t test for data validation, authentication, or any other custom requirements.
• No style or 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.

2.3.1 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 Figure 2.3:⁵

$ rails generate scaffold Micropost content:text user_id:integer
      invoke  active_record
      create    db/migrate/20160515211229_create_microposts.rb
      create    app/models/micropost.rb
      invoke    test_unit
      create      test/models/micropost_test.rb
      create      test/fixtures/microposts.yml
      invoke  resource_route
       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/controllers/microposts_controller_test.rb
      invoke    helper
      create      app/helpers/microposts_helper.rb
      invoke      test_unit
      invoke    jbuilder
      create      app/views/microposts/index.json.jbuilder
      create      app/views/microposts/show.json.jbuilder
      invoke  assets
      invoke    coffee
      create      app/assets/javascripts/microposts.coffee
      invoke    scss
      create      app/assets/stylesheets/microposts.scss
      invoke  scss
   identical    app/assets/stylesheets/scaffolds.scss

(If you get an error related to Spring, just run the command again.) To update our database with the new data model, we need to run a migration as in Section 2.2:

$ rails 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 Section 2.2.1. As you might guess, the scaffold generator has updated the Rails routes file with a rule for Microposts resource, as seen in Listing 2.11.⁶ As with users, the resources :microposts routing rule maps micropost URLs to actions in the Microposts controller, as seen in Table 2.3.

Rails.application.routes.draw do
  resources :microposts
  resources :users
  root 'users#index'
end

The Microposts controller itself appears in schematic form in Listing 2.12. Note that, apart from having MicropostsController in place of UsersController, Listing 2.12 is identical to the code in Listing 2.7. This is a reflection of the REST architecture common to both resources.

class MicropostsController < ApplicationController
  .
  .
  .
  def index
    .
    .
    .
  end

  def show
    .
    .
    .
  end

  def new
    .
    .
    .
  end

  def edit
    .
    .
    .
  end

  def create
    .
    .
    .
  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 Figure 2.12.

Figure 2.12: The new micropost page (/microposts/new).

At this point, go ahead and create a micropost or two, taking care to make sure that at least one has a user_id of 1 to match the id of the first user created in Section 2.2.1. The result should look something like Figure 2.13.

Figure 2.13: The micropost index page (/microposts).

2.3.2 Putting the micro in microposts

Any micropost 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 (à 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 Listing 2.13.

class Micropost < ApplicationRecord
  validates :content, length: { maximum: 140 }
end

The code in Listing 2.13 may look rather mysterious—we’ll cover validations more thoroughly starting in Section 6.2—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 Figure 2.14, Rails renders error messages indicating that the micropost’s content is too long. (We’ll learn more about error messages in Section 7.3.3.)

Figure 2.14: Error messages for a failed micropost creation.

2.3.3 A 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 Listing 2.14 and Listing 2.15.

class User < ApplicationRecord
  has_many :microposts
end

class Micropost < ApplicationRecord
  belongs_to :user
  validates :content, length: { maximum: 140 }
end

We can visualize the result of this association in Figure 2.15. Because of the user_id column in the microposts table, Rails (using Active Record) can infer the microposts associated with each user.

Figure 2.15: The association between microposts and users.

In Chapter 13 and Chapter 14, we will use the association of users and microposts both to display all of 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: "2016-05-15 02:01:31", updated_at: "2016-05-15 02:01:31">
>> first_user.microposts
=> [#<Micropost id: 1, content: "First micropost!", user_id: 1, created_at:
"2016-05-15 02:37:37", updated_at: "2016-05-15 02:37:37">, #<Micropost id: 2,
content: "Second micropost", user_id: 1, created_at: "2016-05-15 02:38:54",
updated_at: "2016-05-15 02:38:54">]
>> micropost = first_user.microposts.first    # Micropost.first would also work.
=> #<Micropost id: 1, content: "First micropost!", user_id: 1, created_at:
"2016-05-15 02:37:37", updated_at: "2016-05-15 02:37:37">
>> micropost.user
=> #<User id: 1, name: "Michael Hartl", email: "michael@example.org",
created_at: "2016-05-15 02:01:31", updated_at: "2016-05-15 02:01:31">
>> exit

(I include exit in the last line just to demonstrate how to exit the console. On most systems, you can also use Ctrl-D for the same purpose.)⁸ 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 Chapter 13 and Chapter 14.

Exercises

Solutions to exercises are available for free with any Rails Tutorial purchase. To see other people’s answers and to record your own, join the Learn Enough Society at learnenough.com/society.

1. Edit the user show page to display the content of the user’s first micropost. (Use your technical sophistication (Box 1.1) to guess the syntax based on the other content in the file.) Confirm by visiting /users/1 that it worked.
2. The code in Listing 2.16 shows how to add a validation for the presence of micropost content in order to ensure that microposts can’t be blank. Verify that you get the behavior shown in Figure 2.16.
3. Update Listing 2.17 by replacing FILL_IN with the appropriate code to validate the presence of name and email attributes in the User model (Figure 2.17).

Listing 2.16: Code to validate the presence of micropost content. app/models/micropost.rb

class Micropost < ApplicationRecord
  belongs_to :user
  validates :content, length: { maximum: 140 },
                      presence: true
end

Figure 2.16: The effect of a micropost presence validation.

Listing 2.17: Adding presence validations to the User model. app/models/user.rb

class User < ApplicationRecord
  has_many :microposts
  validates FILL_IN, presence: true    # Replace FILL_IN with the right code.
  validates FILL_IN, presence: true    # Replace FILL_IN with the right code.
end

Figure 2.17: The effect of presence validations on the User model.

2.3.4 Inheritance hierarchies

We end our discussion of the toy 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), particularly classes. Don’t worry if it’s confusing for now; we’ll discuss these ideas more thoroughly in Section 4.4.

We start with the inheritance structure for models. Comparing Listing 2.18 and Listing 2.19, we see that both the User model and the Micropost model inherit (via the left angle bracket <) from ApplicationRecord, which in turn inherits from ActiveRecord::Base, which is the base class for models provided by Active Record; a diagram summarizing this relationship appears in Figure 2.18. 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 so on.

class User < ApplicationRecord
  .
  .
  .
end

class Micropost < ApplicationRecord
  .
  .
  .
end

Figure 2.18: The inheritance hierarchy for the User and Micropost models.

The inheritance structure for controllers is essentially the same as that for models. Comparing Listing 2.20 and Listing 2.21, we see that both the Users controller and the Microposts controller inherit from the Application controller. Examining Listing 2.22, we see that ApplicationController itself inherits from ActionController::Base, which is the base class for controllers provided by the Rails library Action Pack. The relationships between these classes is illustrated in Figure 2.19.

class UsersController < ApplicationController
  .
  .
  .
end

class MicropostsController < ApplicationController
  .
  .
  .
end

class ApplicationController < ActionController::Base
  .
  .
  .
end

Figure 2.19: The inheritance hierarchy for the Users and Microposts controllers.

As with model inheritance, both the Users and Microposts controllers gain a large amount of functionality by inheriting from a base class (in this case, ActionController::Base), including 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 Section 9.1 we’ll see how to include helpers for logging in and logging out of all of the sample application’s controllers.

2.3.5 Deploying the toy app

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

$ git status
$ git add -A
$ git commit -m "Finish toy 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 toy app to Heroku as in Section 1.5:

$ git push heroku

(This assumes you created the Heroku app in Section 2.1. Otherwise, you should run heroku create and then git push heroku master.)

To get the application’s database to work, you’ll also have to migrate the production database, which involves running the migration command from Listing 2.4 prefixed with heroku run:

$ heroku run rails db:migrate

This updates the database at Heroku with the necessary user and micropost data models. After running the migration, you should be able to use the toy app in production, with a real PostgreSQL database back-end (Figure 2.20).⁹

Finally, if you completed the exercises in Section 2.3.3.1, you will have to remove the code to display the first user’s micropost in order to get the app to load properly. In this case, simply delete the offending code, make another commit, and push again to Heroku.

Figure 2.20: Running the toy app in production.

2.4.1 What we learned in this chapter

• Scaffolding automatically creates code to model data and interact with it through the web.
• Scaffolding is good for getting started quickly but is bad for understanding.
• Rails uses the Model-View-Controller (MVC) pattern for structuring web applications.
• As interpreted by Rails, the REST architecture includes a standard set of URLs and controller actions for interacting with data models.
• Rails supports data validations to place constraints on the values of data model attributes.
• Rails comes with built-in functions for defining associations between different data models.
• We can interact with Rails applications at the command line using the Rails console.