Rails Routing Basics

Learning Goals

Understand the flow of data in a Rails app
Map routes to controller actions
Generate a controller
Send JSON data as a response

Setup

Fork and clone this repo, then run:

$ bundle install
$ rails db:migrate db:seed

This will download all the dependencies for our app and set up the database.

Routing

How does your application know what code to run when it receives a request? This is where routing comes in.

Before we dive into the code and routing configurations, it helps to know how HTTP works at a high level. Below is the flow that takes place when a user attempts to go to a page on a Rails application:

A client sends a request to the server (this could be: a user entering a URL in a browser; a JavaScript application using fetch; etc)
That request is sent to the server where the application's router interprets the request and sends a message to the controller mapped to that route
The controller uses the model to access data from the database
The controller then uses that data to render a view (HTML or JSON)
The server returns an HTTP response, which contains the HTML or JSON data

Creating a Route

Let's try this out in our application. We'll continue using our Cheese Shop app as an example. Our goal will be to create an endpoint for our application that will return a list of all our cheeses as JSON data.

To begin, start up the Rails server with rails s and go to http://localhost:3000/cheeses. As you will see, this throws a routing error: No route matches [GET] "/cheeses". To fix this, we'll need to add a route.

Start by opening the config/routes.rb file and adding the following route inside of the draw block:

get 'cheeses', to: 'cheeses#index'

Let's look at the components that make up this route code:

HTTP verb: in this case we're using the get HTTP verb.
Path: 'cheeses' represents the path in the URL bar that the route will be mapped to.
Controller Action: 'cheeses#index' tells the Rails routing system that this route should be passed through the CheesesController's index action. If the term action sounds foreign, actions are just Rails-speak for an instance method in a controller. So in the CheesesController will be a method called index that gets called when a user goes to /cheeses.

Now go back to http://localhost:3000/cheeses, and refresh the page. You should now see that the error message has changed. It's no longer complaining about not having a route; it should now say: uninitialized constant CheesesController.

Let's fix this by creating a new controller for our cheeses. You can generate a controller using a Rails generator, just like with a model:

$ rails g controller Cheeses --no-test-framework

Notice the naming convention we're following: for a Cheese model, we need a Cheeses controller. Model names are always singular, and controller names are plural. These conventions are important to keep in mind!

This will create a blank controller file /app/controllers/cheeses_controller.rb that we can use to map to the routing file. Since there are a number of methods built into the Rails controller system, you will also want the controller to inherit from the application controller. The new file should have code that looks like this:

class CheesesController < ApplicationController
end

The standard naming convention for controllers is the name of the controller followed by the word Controller.

If you refresh the browser now, you will see a new error: The action 'index' could not be found for CheesesController. This means that it found our controller (woot!) but couldn't find the action index in that controller (womp womp).

We're making good progress (even though we're using EDD - error driven development), and it's good to see each of the errors so that when you encounter these in your real world projects you will know how to fix them. This current error is fixed by adding the following method in the CheesesController:

def index
end

In this method, our goal is to send back a list of all our cheeses as JSON data. To help accomplish our goal, let's do a bit of debugging in our controller action using byebug:

def index
  byebug
end

If you haven't encountered byebug before, it's a way to add breakpoints to our Rails code so we can pause the execution of our code and experiment. It functions similarly to binding.pry.

Visit http://localhost:3000/cheeses in the browser again, and check your terminal: you should be in the byebug session now! That means you have access to everything that's available inside our controller and can try out the code we need to return our list of cheeses.

From your byebug session, run:

cheeses = Cheese.all

This will create a new cheeses variable with all the cheese data from the database!

To see how to return a response with this cheese data, you can also run:

render json: cheeses

Calling the render method with the json: option will produce a JSON-formatted string of data which the controller can then use as the body of the response being sent back to the client. If we pass an Active Record object (in this case, our cheeses variable) to this render method, it will be serialized as JSON data based on the attributes of the object.

To exit byebug gracefully, type continue or c and hit enter.

Now that we've seen how that code will run in byebug, let's add it to the controller action:

def index
  cheeses = Cheese.all
  render json: cheeses
end

Visit http://localhost:3000/cheeses in the browser one more time. Now you should see the JSON data being returned!

Conclusion

You should now have a firm understanding of how to implement basic routing in your application for static pages. Try experimenting by adding more routes and controller actions, and see what other responses you can set up by changing how you are accessing data from the database (like using the .order or .limit methods from Active Record).

Check For Understanding

Before you move on, make sure you can answer the following questions:

When an HTTP request is made to a Rails app, what is the sequence of events that happens to return the response?
What three components make up a route in Rails?

Resources

Rails Routing

Dev-GNM/phase-4-rails-routing-basics-readme