In the last article I have posted how to get started on rails7.

How to get started on rails7

In this article I will tell how to add a model on rails7.

A model is a Ruby class that is used to represent data. Additionally, models can interact with the application's database through a feature of Rails called Active Record.

To define a model, we will use the model generator:

 $ bin/rails generate model Article title:string body:text

Here we define a model whose name is Article. Remember this name, it's important.

After then we need to migrate the records into database by running:

 $ bin/rails db:migrate

How to use the model object (Article)? Let's run the interactive shell on rails.

 $ bin/rails console

In this shell we can access the Article object directly and create a record.

 > article = Article.new(title: "Hello Rails", body: "I am on Rails!")

Don't forget to save the record to database.

 > article.save

How to find the specified record in database? Try this way:

 > Article.find(1)
   Article Load (0.3ms)  SELECT "articles".* FROM "articles" WHERE "articles"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
 =>                                                                 
 #<Article:0x000055cda9dfb480                                       
  id: 1,                                                            
  title: "hello rails",                                             
  body: "I am on rails",                                            
  created_at: Tue, 20 Sep 2022 05:47:56.311534000 UTC +00:00,       
  updated_at: Tue, 20 Sep 2022 05:47:56.311534000 UTC +00:00>

Find out all records in database:

 > Article.all
   Article Load (0.1ms)  SELECT "articles".* FROM "articles"
 =>                                                                                
 [#<Article:0x000055cda9d8d250                                                     
   id: 1,                                                                          
   title: "hello rails",                                                           
   body: "I am on rails",                                                          
   created_at: Tue, 20 Sep 2022 05:47:56.311534000 UTC +00:00,                     
   updated_at: Tue, 20 Sep 2022 05:47:56.311534000 UTC +00:00>,                    
  #<Article:0x000055cda9d8d188                                                     
   id: 2,                                                                          
   title: "welcome buddy",                                                         
   body: "how are you doing today?",                                               
   created_at: Tue, 20 Sep 2022 05:50:19.850369000 UTC +00:00,                     
   updated_at: Tue, 20 Sep 2022 05:50:19.850369000 UTC +00:00>

Now the records are saved into database. We want to show them in pages.

First, the controller "app/controllers/articles_controller.rb" would have to list all records, by this way:

 class ArticlesController < ApplicationController
   def index
     @articles = Article.all
   end
 end

Here @articles is an instance variable which can be accessed by views.

The view file "app/views/articles/index.html.erb" for index action will be changed as following.

 <h1>Articles</h1>
 <ul>
   <% @articles.each do |article| %>
     <li>
       <a href="/articles/<%= article.id %>">
         <%= article.title %>
       </a>
     </li>
   <% end %>
 </ul>

The above code is a mixture of HTML and ERB. ERB is a templating system that evaluates Ruby code embedded in a document. Here, we can see two types of ERB tags: <% %> and <%= %>. The <% %> tag means "evaluate the enclosed Ruby code." The <%= %> tag means "evaluate the enclosed Ruby code, and output the value it returns."

Wait, in above code we have added "/articles/id" path, but we didn't define a route for it.

So we have to add a route for that path. Edit "config/routes.rb" and change it to:

 Rails.application.routes.draw do
   root "articles#index"
   get "/articles", to: "articles#index"
   get "/articles/:id", to: "articles#show"
 end

In the route above we have added "show" action, we have to add this acton in controller too.

Edit "app/controllers/articles_controller.rb" and change it to:

 class ArticlesController < ApplicationController
   def index
     @articles = Article.all
   end
   def show
     @article = Article.find(params[:id])
   end
 end

And, we want to add a erb template for "show" action. Create "app/views/articles/show.html.erb" with the content:

 <h1><%= @article.title %></h1>
 <p><%= @article.body %></p>

All done. When access to http://www.sample.com:3000/ you will see the content as:

As you see, the full process for generating a web page is: route -> controller -> model -> view.

  • The browser makes a request: GET http://localhost:3000.
  • Our Rails application receives this request.
  • The Rails router maps the root route to the index action of ArticlesController.
  • The index action uses the Article model to fetch all articles in the database.
  • Rails automatically renders the app/views/articles/index.html.erb view.
  • The ERB code in the view is evaluated to output HTML.
  • The server sends a response containing the HTML back to the browser.

For more details please reference the official doc.

Getting started with Rails