Review of the Rails MVC

Learning Goals

• Review the MVC structure of Rails

Introduction

We left Rails with the ability to manage data and display it in a browser using models, views and controllers. As we have seen, Rails can entirely handle the front and backend of a website - one reason it is a great tool for web development.

With Rails, though, we aren't required to strictly render ERB views. In this code-along, we're going to briefly review the MVC structure of Rails as well as rendering through the Rails controller. The basic files of a Rails app are provided in this lesson, but some files will need content from this Readme to function.

Review of MVC Structure

The model, view, controller structure is a separation of concerns where groups of files have specific jobs and interact with each other in very defined ways:

• Models: The 'logic' of a web application. This is where data is manipulated and/or saved to a database.
• Views: The 'frontend', user-facing part of a web application - this is the only part of the app that the user interacts with directly. Views generally consist of HTML, CSS, and Javascript.
• Controllers: The go-between for models and views. The controller relays data from the browser to the application, and from the application to the browser.

To review the model, view, controller structure in Rails, we're going to quickly walk through the setting up a basic resource.

Let's imagine we want to build an amateur bird watching website. To start, we'll just try to create a site that displays different types of birds. Using Rails and this MVC pattern, the data about these birds would be contained within a database, so let's set that up.

First, we need a migration to set up the database:

# db/migrate/2019_create_birds.rb

class CreateBirds < ActiveRecord::Migration[5.2]
  def change
    create_table :birds do |t|
      t.string :name
      t.string :species

      t.timestamps
    end
  end
end

Then run rails db:migrate to create the schema. We set this up first because we quickly move away from working directly with the database. With Rails, we actually want to create a model to represent and manipulate the data. Create the following Bird model in app/models/bird.rb:

class Bird < ApplicationRecord
end

ASIDE: By inheriting from ApplicationRecord, Bird also inherits from ActiveRecord, which you may remember is an ORM, or Object Relational Map. Because of this, we gain many useful methods like all and save without having to include any additional methods.

With a model and the database set up, we can add a little data to help display our view later. Some seed data is provided in db/seeds.rb; just run rails db:seed and it should create four Bird records. You can always check these by running rails console, then use Bird.all to confirm these instances are appearing.

Now we can configure a route and corresponding controller method. To keep things simple, We'll just set up a basic index action with a route:

Rails.application.routes.draw do
  get '/birds' => 'birds#index'
end

And a controller with corresponding action:

class BirdsController < ApplicationController
  def index
    @birds = Bird.all
  end
end

Any visitors to '/birds' will get routed to the index action in the BirdsController. This action shows one thing - get all instances of the Bird model and store them in a variable, @birds. Now the controller and model are set up to work together.

Rails favors convention over configuration. For this reason, if a folder and file are present in the views folder that correspond to a controller and action listed on a route, Rails will display that view by default.

In our example, we have a route pointed to 'birds#index'. In app/views, we also have a /birds folder containing index.html.erb. Rails recognizes this as a match and so implicitly renders this file. This is the same as writing:

class BirdsController < ApplicationController
  def index
    @birds = Bird.all
    render 'birds/index.html.erb'
  end
end

The provided birds/index.html.erb file contains ERB code that will list out each bird's name and species in an unordered list:

<h1>Birds</h1>

<ul>
<% @birds.each do |bird| %>
    <li><%= bird.name %> - <%= bird.species %></li>
<% end %>
</ul>

Conclusion

As it is currently configured, we already have a Rails app up and running using the MVC structure! If you run rails server and visit the '/birds' path, you should see a list of birds.

With minimal work, as we just saw, we were able to spin up a resource backed by a database and serve it up in a browser. In short, when a visitor goes to '/birds' on this Rails app, the controller retrieves data from the Bird model and then serves that data to the visitor by displaying it in a view.

So where does JavaScript fit in? Well, we have the skills to build out our own frontends. As we will see in the next lesson, rather than using the ERB view, Rails is flexible enough to give us something we can use with JavaScript and fetch().

Resources

• ActiveRecord Basics
• Layouts and Rendering in Rails