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Introduction to Ruby on Rails

This is the reference document used in the "Introduction to Ruby on Rails" Hackerschool workshop AY19/20 Semester 1.

Made by Herbert Ilhan Tanujaya, for NUS Hackers.

Preparation

Check

ruby -v returns

ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]

(OK as long as it’s Ruby 2.6)

rails -v returns

Rails 6.0.0

The Model-View-Controller Framework

Rails applications are structured with the Model-View-Controller (MVC) framework. This is very important! You really need to understand this MVC framework before you can understand Ruby on Rails.

The Three Components

  • Model: the objects that is central to the application
  • Controller: actions you can do on the model
  • View: the interface the user sees

In general, the model stands by itself, and the controller and the view are linked together.

The controller and the view acts upon a model. One model can have many controller actions / views.

Using Reddit as a Sample

In Reddit, we can create posts and view posts. This post by itself is a model.

List posts

We note that we can view posts. Hence, "view" is a controller action associated with the post model. This picture above is a screenshot of the corresponding "view" controller.

Show post

We can also show single posts. Hence, "show" is also a controller action associated with the post model. This picture above is a screenshot of the corresponding "show" controller.

New post

Of course, we can also create posts. Hence, "create" is a controller action associated with the post model. The screenshot above is the form that we fill in to create posts. However, the form itself is not associated with the "create" action -- in this view, we are not creating anything yet! We are only filling in the form to create the post.

Hence, in fact there are 2 separate controller actions here: the first one is to show the form that we can fill in to create a post; and the second one is the action that actually creates the post (after we click the submit button). We can name the first one, the one that only shows the form, to be the "new" action. This "new" action has a view associated with it. Meanwhile, the action that actually creates the post shall be named as the "create" action. Do note that there is no view associated with the "create" action: once we create the post, we get redirected to the "view posts" view directly.

We can also name other actions associated with posts: edit, delete, upvote, downvote, and so on. We can also observe other models, such as comments, subreddits, and so on. These other models also have their own controllers and views.

Architecture

Roughly, the architecture of a generic Rails application is like this:

Rails architecture

Creating a Book Shop App

rails new book-shop

will create a bunch of files, and install things.

Afterwards, you can cd book-shop

$ ls
app  babel.config.js  bin  config  config.ru  db  Gemfile  Gemfile.lock  lib
log  node_modules  package.json  postcss.config.js  public  Rakefile  README.md
storage  test  tmp  vendor  yarn.lock

Checking that the App is Created

$ rails server
=> Booting Puma
=> Rails 6.0.0 application starting in development
=> Run `rails server --help` for more startup options
Puma starting in single mode...
* Version 3.12.1 (ruby 2.6.3-p62), codename: Llamas in Pajamas
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://localhost:3000
Use Ctrl-C to stop

Open localhost:3000 in your browser. You should see "Yay! You're on Rails!"

Defining our Requirements and Model

  • The book store contains books.
  • Every book has a title, an author, and a book count (number of books remaining).
  • Everyone can view a listing of all books, as well as a single book itself.
  • For now, let us assume everyone can create, update, and delete books as well.
  • Everyone can "purchase" a book, reducing its count by 1. You cannot purchase if the book count is zero. Book model diagram

Creating our Model

$ rails generate model Book title:string author:string count:integer
Running via Spring preloader in process 16753
      invoke  active_record
      create    db/migrate/20190728121953_create_books.rb
      create    app/models/book.rb
      invoke    test_unit
      create      test/models/book_test.rb
      create      test/fixtures/books.yml

Then app/models/book.rb should contain:

class Book < ApplicationRecord
end

Running a Database Migration

$ rails db:migrate
== 20190728123042 CreateBooks: migrating ======================================
-- create_table(:books)
   -> 0.0010s
== 20190728123042 CreateBooks: migrated (0.0011s) =============================

Manipulating the Models through the Rails Console

$ rails console
Running via Spring preloader in process 30584
Loading development environment (Rails 6.0.0)
irb(main):001:0>

You can type Ruby on it.

irb(main):001:0> 1234 + 5678
=> 6912
irb(main):002:0> 'xa' * 7
=> "xaxaxaxaxaxaxa"

Creating Books

irb(main):001:0> book = Book.new()
   (0.3ms)  SELECT sqlite_version(*)
=> #<Book id: nil, title: nil, author: nil, count: nil, created_at: nil, updated_at: nil>
irb(main):002:0> book.title = 'Introduction to Algorithms'
=> "Introduction to Algorithms"
irb(main):003:0> book.author = 'Cormen, Leiserson, Rivest, Stein'
=> "Cormen, Leiserson, Rivest, Stein"
irb(main):004:0> book.count = 100
=> 100
irb(main):005:0> book.save()
   (0.2ms)  begin transaction
  Book Create (0.6ms)  INSERT INTO "books" ("title", "author", "count",
  "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["title", "Introduction
  to Algorithms"], ["author", "Cormen, Leiserson, Rivest, Stein"], ["count",
  100], ["created_at", "2019-09-13 08:12:49.508990"], ["updated_at",
  "2019-09-13 08:12:49.508990"]]
   (13.8ms)  commit transaction
=> true

Book Create indicates that books are being inserted to the database.

Information can also be supplied to the new method:

irb(main):006:0> gatsby = Book.new(title: 'The Great Gatsby', author: 'Fitzgerald', count: 20)
=> #<Book id: nil, title: "The Great Gatsby", author: "Fitzgerald", count: 20,
created_at: nil, updated_at: nil>
irb(main):007:0> gatsby.save()
   (0.1ms)  begin transaction
  Book Create (0.6ms)  INSERT INTO "books" ("title", "author", "count", "created_at",
  "updated_at") VALUES (?, ?, ?, ?, ?)  [["title", "The Great Gatsby"],
  ["author", "Fitzgerald"], ["count", 20], ["created_at", "2019-07-28 12:37:02.850518"],
  ["updated_at", "2019-07-28 12:37:02.850518"]]
   (8.0ms)  commit transaction
=> true

Or do it in one shot with the create method:

irb(main):008:0> Book.create(title: 'Guns, Germs, and Steel', author: 'Diamond', count: 20)
   (0.1ms)  begin transaction
  Book Create (0.6ms)  INSERT INTO "books" ("title", "author", "count", "created_at",
  "updated_at") VALUES (?, ?, ?, ?, ?)  [["title", "Guns, Germs, and Steel"],
  ["author", "Diamond"], ["count", 20], ["created_at", "2019-07-28 12:37:32.049849"],
  ["updated_at", "2019-07-28 12:37:32.049849"]]
   (10.3ms)  commit transaction
=> #<Book id: 3, title: "Guns, Germs, and Steel", author: "Diamond", count: 20,
created_at: "2019-07-28 12:37:32", updated_at: "2019-07-28 12:37:32">

Getting Information on Books

Viewing All Books

irb(main):001:0> Book.all()
  Book Load (0.5ms)  SELECT  "books".* FROM "books" LIMIT ?  [["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<Book id: 1, title: "Introduction to Algorithms",
author: "Cormen, Leiserson, Rivest, Stein", count: 100, created_at: "2019-07-28 12:35:08",
updated_at: "2019-07-28 12:35:08">, #<Book id: 2, title: "The Great Gatsby",
author: "Fitzgerald", count: 20, created_at: "2019-07-28 12:37:02",
updated_at: "2019-07-28 12:37:02">, #<Book id: 3, title: "Guns, Germs, and Steel",
author: "Diamond", count: 20, created_at: "2019-07-28 12:37:32",
updated_at: "2019-07-28 12:41:08">]>

Getting a Subset of Books

irb(main):002:0> Book.where(title: 'Introduction to Algorithms')
  Book Load (0.5ms)  SELECT  "books".* FROM "books" WHERE "books"."title" = ?
  LIMIT ?  [["title", "Introduction to Algorithms"], ["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<Book id: 1, title: "Introduction to Algorithms",
author: "Cormen, Leiserson, Rivest, Stein", count: 100, created_at: "2019-07-28
12:35:08", updated_at: "2019-07-28 12:35:08">]>
irb(main):003:0> Book.where(title: 'No title')
  Book Load (0.3ms)  SELECT  "books".* FROM "books" WHERE "books"."title" = ?
  LIMIT ?  [["title", "No title"], ["LIMIT", 11]]
=> #<ActiveRecord::Relation []>
irb(main):004:0> Book.where(count: 20)
  Book Load (0.4ms)  SELECT  "books".* FROM "books" WHERE "books"."count" = ?
  LIMIT ?  [["count", 20], ["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<Book id: 2, title: "The Great Gatsby", author:
"Fitzgerald", count: 20, created_at: "2019-07-28 12:37:02", updated_at:
"2019-07-28 12:37:02">, #<Book id: 3, title: "Guns, Germs, and Steel", author:
"Diamond", count: 20, created_at: "2019-07-28 12:37:32", updated_at:
"2019-07-28 12:41:08">]>

Getting a specific book

irb(main):005:0> Book.find_by(author: 'Diamond')
  Book Load (0.4ms)  SELECT  "books".* FROM "books" WHERE "books"."author" = ?
  LIMIT ?  [["author", "Diamond"], ["LIMIT", 1]]
=> #<Book id: 3, title: "Guns, Germs, and Steel", author: "Diamond", count: 20,
created_at: "2019-07-28 12:37:32", updated_at: "2019-07-28 12:41:08">
irb(main):006:0> Book.find_by(count: 20)
  Book Load (0.3ms)  SELECT  "books".* FROM "books" WHERE "books"."count" = ?
  LIMIT ?  [["count", 20], ["LIMIT", 1]]
=> #<Book id: 2, title: "The Great Gatsby", author: "Fitzgerald", count: 20,
created_at: "2019-07-28 12:37:02", updated_at: "2019-07-28 12:37:02">

Manipulating Books

irb(main):008:0> gatsby = Book.find_by(title: 'The Great Gatsby')
  Book Load (0.3ms)  SELECT  "books".* FROM "books" WHERE "books"."title" = ?
  LIMIT ?  [["title", "The Great Gatsby"], ["LIMIT", 1]]
=> #<Book id: 2, title: "The Great Gatsby", author: "Fitzgerald", count: 20,
created_at: "2019-07-28 12:37:02", updated_at: "2019-07-28 12:37:02">
irb(main):009:0> gatsby.count = 15
=> 15
irb(main):010:0> gatsby.save()
   (0.2ms)  begin transaction
  Book Update (0.4ms)  UPDATE "books" SET "count" = ?, "updated_at" = ? WHERE
  "books"."id" = ?  [["count", 15], ["updated_at", "2019-07-28
  12:43:12.771246"], ["id", 2]]
   (11.0ms)  commit transaction
=> true

Alternatively, do it in one shot with the update method:

irb(main):011:0> gatsby.update(count: 10)
   (0.1ms)  begin transaction
  Book Update (0.6ms)  UPDATE "books" SET "count" = ?, "updated_at" = ? WHERE
  "books"."id" = ?  [["count", 10], ["updated_at", "2019-07-28
  12:44:15.082445"], ["id", 2]]
   (10.5ms)  commit transaction
=> true

Deleting Books

irb(main):012:0> gatsby.destroy()
   (0.1ms)  begin transaction
  Book Destroy (0.4ms)  DELETE FROM "books" WHERE "books"."id" = ?  [["id", 2]]
   (8.0ms)  commit transaction
=> #<Book id: 2, title: "The Great Gatsby", author: "Fitzgerald", count: 10,
created_at: "2019-07-28 12:37:02", updated_at: "2019-07-28 12:44:15">
irb(main):013:0> Book.all()
  Book Load (0.3ms)  SELECT  "books".* FROM "books" LIMIT ?  [["LIMIT", 11]]
=> #<ActiveRecord::Relation [#<Book id: 1, title: "Introduction to Algorithms",
author: "Cormen, Leiserson, Rivest, Stein", count: 100, created_at: "2019-07-28
12:35:08", updated_at: "2019-07-28 12:35:08">, #<Book id: 3, title: "Guns,
Germs, and Steel", author: "Diamond", count: 20, created_at: "2019-07-28
12:37:32", updated_at: "2019-07-28 12:41:08">]>

Adding Controllers and Views

Notice the plural Books!

$ rails generate controller Books
Running via Spring preloader in process 16928
      create  app/controllers/books_controller.rb
      invoke  erb
      create    app/views/books
      invoke  test_unit
      create    test/controllers/books_controller_test.rb
      invoke  helper
      create    app/helpers/books_helper.rb
      invoke    test_unit
      invoke  assets
      invoke    scss
      create      app/assets/stylesheets/books.scss

Let us do all of CRUD: Create, Read, Update, and Delete.

List

Change app/controllers/books_controller.rb to:

class BooksController < ApplicationController
  def list
    @books = Book.all()
  end
end

Let's create a view that corresponds to this list controller. In app/views/books/list.html.erb:

<% @books.each do |book| %>
  <p>ID: <%= book.id %>, title: <%= book.title %>, author: <%= book.author %>,
  count: <%= book.count %></p>
<% end %>

Let us make this page accessible from the web. In config/routes.rb:

Rails.application.routes.draw do
  get('books', to: 'books#list')
end

Now, if you run rails server and open localhost:3000/books/ you should be able to see the list of all books.

What is actually happening?

  • @books defined in the controller is shared to the view
  • .each do |book| is like a for-loop for Ruby that iterates through the @books array
  • <% ... %> is a ERB tag. <%= ... %> is also an ERB tag, but it evaluates it and prints it to the HTML.
  • get('books', to: 'books#list') points /books to the Books controller, list method

You can try these things:

  • Change @books to e.g. Book.where(author: 'Diamond')
  • Change @books to books, without the at sign
  • Make your views easier to read, maybe by adding line breaks, or even with tables
  • Change your route definition to maybe get('aaa', ...) or something

Show

Add this show method to app/controllers/books_controller.rb:

def show
  id = params[:id]
  @book = Book.find_by(id: id)
end

In app/views/books/show.html.erb:

<p>Title: <%= @book.title %></p>
<p>Author: <%= @book.author %></p>
<p>Count: <%= @book.count %></p>

In config/routes.rb:

get('books/:id', to: 'books#show')

Then open e.g. localhost:3000/books/1. You should be able to see your book.

What is actually happening?

  • By defining the routes to books/:id, we "capture" the :id parameter.
  • params[:id] accesses that parameter in the controller.
  • :id is what we call a symbol. Just assume that it is a string.

Create

We need new and create.

  • new is when you get the form for people to fill in book information
  • create is when you actually submit the form to create the object

New action

In the controller:

def new
  @book = Book.new()
end

In app/views/book/new.html.erb:

<%= form_with(model: @book, local: true) do |form| %>
  <p>
    <%= form.label(:title) %><br>
    <%= form.text_field(:title) %>
  </p>

  <p>
    <%= form.label(:author) %><br>
    <%= form.text_field(:author) %>
  </p>

  <p>
    <%= form.label(:count) %><br>
    <%= form.text_field(:count) %>
  </p>

  <p>
    <%= form.submit() %>
  </p>
<% end %>

In the routes:

Rails.application.routes.draw do
  get('books/new', to: 'books#new')
  get('books', to: 'books#list')
  get('books/:id', to: 'books#show')
end

Make sure that books/new is before books/:id, so that it has higher priority!

Now visit localhost:3000/books/new.

What is actually happening?
  • Again, with the books/new routes defined to books#new, it goes into the new action.
  • form_with is a form helper. It is a Rails specific function that helps you generate forms. Inspect the HTML to see what is actually being generated. You pass in a model, in this case @book.
  • You can pass in a Ruby block that shows how the HTML is being generated. This block takes in form, and you can access objects from the form's attributes.

Adding link to new book

In the list view, add:

<%= link_to('New book', books_new_path) %>
What is actually happening?
  • books_new_path is automatically defined by Rails:
$ rails routes
   Prefix Verb URI Pattern          Controller#Action
books_new GET  /books/new(.:format) books#new
    books GET  /books(.:format)     books#list
          GET  /books/:id(.:format) books#show
...

You need to append _path to the "Prefix" column to get your paths. Tip: Rails's error pages also show this helpful table. Try e.g. localhost:3000/asdf.

  • link_to is a helper function provided by Rails.

Create action

In the controller:

def create
  book = Book.new(book_params())

  if book.save()
    redirect_to(books_path)
  else
    redirect_to(books_new_path)
  end
end

private
def book_params
  params.require(:book).permit(:title, :author, :count)
end

In the routes:

Rails.application.routes.draw do
  get('books/new', to: 'books#new')
  post('books', to: 'books#create')
  get('books', to: 'books#list')
  get('books/:id', to: 'books#show')
end

Now you can create a book!

What is actually happening?

A lot is happening here.

  • Let us disect the method book_params. It takes params, which behaves like a hash (dictionary in Python / HashMap in Java / unordered_map in C++). require requires params to contain a :book.
  • In the :book key, it is another hash. We only extract :title, :author, and :count out.
  • redirect_to redirects your browser to another path.
  • books_path and books_new_path is automatically defined by Rails:
$ rails routes
   Prefix Verb URI Pattern          Controller#Action
books_new GET  /books/new(.:format) books#new
    books GET  /books(.:format)     books#list
          GET  /books/:id(.:format) books#show
...
  • We use book instead of @book, since we do not have anything to share with the views. In fact, we do not have any view associated with the edit action.
  • We do not have any views, since we always redirect somewhere.
  • post in the routes, indicates a POST request. This is different from typical requests your browser makes, which is GET.

You can try these things:

  • puts(params), puts(book_params()), and puts(book) on the controller
How HTTP requests work

Refer to https://developer.mozilla.org/en-US/docs/Web/HTTP/Overview#HTTP_flow.

A request contains the method and the path, and you get some response back. Typically, in a static site, you only give back HTML and CSS. Rails controls what is being sent given a request. In this case we are actually using Puma as our web server.

Edit

Just like new and create, we need edit and update.

  • edit is when you get the form for people to edit book information
  • update is when you actually submit the form to change the object

Controller

def edit
  id = params[:id]
  @book = Book.find_by(id: id)
end

def update
  id = params[:id]
  book = Book.find_by(id: id)

  if book.update(book_params())
    redirect_to(books_path)
  else
    redirect_to(books_edit_path)
  end
end

View

app/views/books/edit.html.erb:

<%= form_with(model: @book, local: true) do |form| %>
  <p>
    <%= form.label(:title) %><br>
    <%= form.text_field(:title) %>
  </p>

  <p>
    <%= form.label(:author) %><br>
    <%= form.text_field(:author) %>
  </p>

  <p>
    <%= form.label(:count) %><br>
    <%= form.text_field(:count) %>
  </p>

  <p>
    <%= form.submit() %>
  </p>
<% end %>

Routes

Rails.application.routes.draw do
  get('books/new', to: 'books#new')
  post('books', to: 'books#create')
  get('books/:id/edit', to: 'books#edit', as: 'book_edit')
  patch('books/:id', to: 'books#update')
  get('books', to: 'books#list')
  get('books/:id', to: 'books#show', as: 'book')
end

Delete

Controller

def destroy
  id = params[:id]
  Book.find_by(id: id).destroy()
  redirect_to(books_path)
end

Routes

Rails.application.routes.draw do
  get('books/new', to: 'books#new')
  post('books', to: 'books#create')
  get('books/:id/edit', to: 'books#edit', as: 'book_edit')
  patch('books/:id', to: 'books#update')
  get('books', to: 'books#list')
  get('books/:id', to: 'books#show', as: 'book')
  delete('books/:id', to: 'books#destroy', as: 'book_destroy')
end

app/views/books/show.html.erb

<p>Title: <%= @book.title %><p>
<p>Author: <%= @book.author %><p>
<p>Count: <%= @book.count %><p>
<p><%= link_to('Delete book', book_destroy_path(@book), method: :delete) %></p>

More!

In general, you can refer to https://guides.rubyonrails.org/getting_started.html. It can be somewhat overwhelming, but it is up to date.

Using routes resources

Replace config/routes.rb with

Rails.application.routes.draw do
  resources(:books)
end

Adding CSS

Check out app/assets/stylesheets/books.scss. SCSS is like CSS but better: https://sass-lang.com/

Let us try to put this in that file: (courtesy of https://blog.koley.in/2019/339-bytes-of-responsive-css)

@import url('https://fonts.googleapis.com/css?family=Fira+Sans:300');
body {
  font-family: 'Fira Sans', sans-serif;
  line-height: 1.6;
  color: #222;
  max-width: 40rem;
  padding: 2rem;
  margin: auto;
  background: #fafafa;
}

In reality, you might want to create a new file for this application-wide style. Try to put only books-related styles in the books.scss file.

Model Validation

Let us change app/models/book.rb:

class Book < ApplicationRecord
  validates(:count, numericality: { greater_than_or_equal_to: 0 })
end

Now you cannot set count to be negative:

irb(main):001:0> Book.find_by(id: 1).update(count: -3)
  Book Load (0.2ms)  SELECT  "books".* FROM "books" WHERE "books"."id" = ?
  LIMIT ?  [["id", 1], ["LIMIT", 1]]
   (0.0ms)  begin transaction
   (0.0ms)  rollback transaction
=> false

Notice the "rollback transaction" (instead of the normal "commit transaction"), and that it returns false instead of true.

Beyond: Convention over Configuration

"Not everyone is a special snowflake." Rails prefers doing things implicitly/magically over explicitness.

Appendix: Deploying your application

Refer to https://devcenter.heroku.com/articles/getting-started-with-rails5.

Appendix: Idiomatic Ruby

  • In general, parantheses on function calls can be omitted
  • The last value of a function in Ruby serves as a return value
  • Think of a block as a function
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