Adding Routes

We’re now pretty proficient in writing Ruby programs, especially with the aid of gems and APIs. Great! However, if we (the developers) are the only ones that can run these programs (from the command line through the ruby interpreter), then they aren’t much use.

It’s time to start adding a web interface on top of our Ruby programs so that external users can interact with and benefit from them.


For an application that runs on a server and transmits information across the internet, the interface consists of a set of URLs that a user can visit. Each URL will either

  • display a page with some information (“get” in HTTP terminology)
  • trigger the storing of some information (“post” in HTTP terminology)
  • trigger the deleting of some information (“delete” in HTTP terminology)
  • trigger the updating of some information (“patch” in HTTP terminology)
  • forward to another URL
  • or some combination of the above

Most obviously, the user might be visiting the URLs in their browser by typing into the address bar or clicking on links. Or, more and more commonly, users might be visiting from native iPhone or Android apps without even knowing that, behind the scenes, they are visiting URLs to store and retrieve the information they need.

But make no mistake: if there is information being stored in a central database, then there’s a web server running somewhere and URLs are being visited with each action a user takes.

You can fully specify a web application by listing out the URLs that users can visit, and what happens when each URL is visited. For example, let’s say we wanted to build an interactive game of Rock, Paper, Scissors. The complete specifications (or specs, for short) for this app might look like this:

  • http://[OUR APP DOMAIN]/rock — Should display “You played rock.”, a random move by the computer, and the outcome.
  • http://[OUR APP DOMAIN]/paper — Should display “You played paper.”, a random move by the computer, and the outcome.
  • http://[OUR APP DOMAIN]/scissors — Should display “You played scissors.”, a random move by the computer, and the outcome .
  • http://[OUR APP DOMAIN]/ — A welcome page that displays
    • “Happy Monday!” (or whatever day it is).
    • The rules of the game.
    • For example,

       Happy Tuesday!
       Rock beats Scissors, Paper beats Rock, Scissors beats Paper.
       Point your browser at /rock, /paper, or /scissors to play the game.

Now — how do we get our web server to perform the above tasks when users visit the above URLs? Ensure that your rails server is running and then try pointing your browser at http://[YOUR APP DOMAIN]/rock. Right now, you should see a “No route matches error.” Let’s fix that.


The key is: we need to connect a visit to a URL by a user to a Ruby method that will perform the desired work and send back a response. We do this by defining routes in a very special file, config/routes.rb, which is included in every Rails app.

Here’s an example routes.rb with two routes defined in it:

# /config/routes.rb

Rails.application.routes.draw do
  get("/rock", { :controller => "application", :action => "play_rock" })

  get("/", { :controller => "application", :action => "homepage" })


  • All of our routes must be contained within the block following Rails.application.routes.draw. A new Rails app will already come with this code pre-written in routes.rb.
  • Each route is comprised of the get method1 and its two arguments:
    • The first argument to get is a String: the path that we want users to be able to visit (the path is the portion of the URL that comes after the domain name).
    • The second argument to get is a Hash: this is where we tell Rails which method to call when a user visits the path in the first argument. (We’ll have to actually write this method in the next step, after we write the route.)

      The Hash must have two key/value pairs:

      • :controller: The value for this key is what we’re going to name the class that contains the method we want Rails to call when the user visits the path. For now we’re going to default this value to "application" — you’ll see why in a minute.
      • :action: The value for this key is the what we’re going to name the method itself. “Action” is the term used to refer to Ruby methods that are triggered by users visiting URLs.


If we add the first route above to our routes.rb and then type in http://[OUR APP DOMAIN]/rock into the browser, the error we’ll see is:

The action 'play_rock' could not be found for ApplicationController

This is good! That means we defined the route correctly. If you still see a “No route matches” error, then double-check your route syntax and get that error to go away before you proceed further.

Now we have to define a method called play_rock within a class called ApplicationController, since that’s what we specified as the value for the :action key back in our route. You can find the class in the app/controllers/ folder, and add the method within:

class ApplicationController < ActionController::Base
  def play_rock

At the end of the day, the job of an action is to send back a response to the user. A response can be either:

  • Rendering some data, in any one of many formats:
  • Plain text.
  • JSON for an application to consume — we’ve seen JSON before (in APIs), and consumed it ourselves with our Ruby scripts.
  • HTML for their browser to draw — we’ll learn this soon.
  • Less commonly, any other format: CSV, PDF, XML, etc.
  • Or, the action can forward the user to another URL. This is known as redirecting.

We get a method for each of these two: render for the first, and redirect_to for the second.


Let’s try redirect_to first:

class ApplicationController < ActionController::Base
  def play_rock

Now visit /rock in your browser and see what happens. As you can see, the argument to redirect_to is a String which contains some URL that you want the user to simply be forwarded to. This will come in handy later when, for example, we want to send the user directly back to a list of all photos after they’ve deleted a photo.


More commonly, however, we’ll use render to send some data to the user’s browser for display:

class ApplicationController < ActionController::Base
  def play_rock
    render({ :plain => "Hello, world!" })

The argument to render is a Hash that specifies how we want to send the data back. We have a lot of options to choose from, corresponding to how many different formats there are in use for transmitting data. :plain is just going to send back plain text, which is enough for us for now.

Try visiting /rock in your browser and you should now see a response rather than an error message. Congratulations! You’ve wired up your very first route; prepare to do it a million more times, because all developers do all day is pick the next spec , wire up the route for the URL so that a user can visit it, and then implement the logic to send back the correct information.

Let’s go ahead and implement the logic for /rock. You can write as much Ruby as you need to in the method prior to the render:

class ApplicationController < ActionController::Base
  def play_rock
    moves = ["rock", "paper", "scissors"]

    computer_move = moves.sample
    if computer_move == "rock"
      outcome = "tied"
    elsif computer_move == "paper"
      outcome = "lost"
    elsif computer_move == "scissors"
      outcome = "won"

    full_message = "You played rock. They played " + computer_move + ". You " + outcome + "!"

    render({ :plain =>  full_message })

Now, every time you visit /rock (or refresh the page), you should see a dynamically generated page. Yay! 🎉

Get some practice at wiring up routes by implementing /paper and /scissors, similarly.

Then, implement the homepage connecting the dots for the route for the path of just plain / (with nothing after the slash).

Once you’ve implemented all four of the specs above, then your job is done!

Addendum: Rendering JSON

Imagine that we wanted to build a native iPhone app that asked our server for some information; in this simple example, for a random computer move and an outcome, but in the real-world things like the local weather given a latitude and a longitude.

Rather than rendering a pre-defined message in plain text, it’s usually more helpful to the iPhone developer to render the data in JSON format, so that they can parse it, easily fetch whichever values they need, and assemble their own interface.

Here is some JSON that would be convenient for an external application to parse:

{ "player_move":"rock", "computer_move":"paper", "outcome":"lost" }

Notice that JSON uses strings as keys — this is because JavaScript doesn’t have the equivalent of Ruby’s Symbol class. Also, there are no hash rockets; JSON just uses colons to separate keys and values.

Fortunately, just as it was easy for us to convert a String containing JSON into Ruby Arrays/Hashes using the JSON.parse method, it is also easy for us to go in the other direction: both Array and Hash have methods called .to_json. Let’s create a Ruby Hash that resembles the JSON above:

response_hash = { :player_move => "rock", :computer_move => "paper", :outcome => "lost" }

We can then convert this into a String in JSON format with .to_json:




The \" represents double-quotes; we need the backslash, known as an “escape”, because we’re already within a double-quoted string and don’t want to terminate it. You can puts the string to see it formatted:

puts response_hash.to_json



Great! That means we can update our action if we want to send back JSON instead:

class ApplicationController < ActionController::Base
  def play_rock
    moves = ["rock", "paper", "scissors"]

    computer_move = moves.sample
    if computer_move == "rock"
      outcome = "tied"
    elsif computer_move == "paper"
      outcome = "lost"
    elsif computer_move == "scissors"
      outcome = "won"

    response_hash = { :player_move => "rock", :computer_move => "paper", :outcome => "lost" }

    render({ :plain =>  response_hash.to_json })

Congratulations — you just built your first API endpoint! 🙌🏾

Addendum: Custom Controller Files

We don’t have to put all of our actions within the default ApplicationController file that comes included with any Rails app; we can add our own controllers, if we want to organize things a bit more. With an app of any non-trivial size, you’ll end up with hundreds of actions, and it can get unwieldy to put them all in one gigantic application_controller.rb.

Instead, we can change our route for /rock to this:

get("/rock", { :controller => "game", :action => "play_rock" })

Now when a user visits /rock, they will see an error uninitialized constant GameController.

As we know, when Ruby says “uninitialized constant” it means “I can’t find that class”.

So, what’s going on here? When we said :controller => "game" in the route, we told Rails to look for a class called GameController when someone visits /rock.

  • All of the controller class names will end in ...Controller, and they will begin with whatever value we provided for the key :controller in the route.
  • Like all Ruby classes, the name must be CamelCase (not snake_case or Some_Hybrid). So in this case, it will be GameController.
  • The class must be defined in a Ruby file that is the snake_cased version of its name. Rails will itself use the .underscore method to figure out the name; we can try it ourselves in rails console:

     [2] pry(main)> "GameController".underscore
     => "game_controller"
  • The Ruby file must be placed within the app/controllers/ folder. So, in this case, we create a file called app/controllers/game_controller.rb (don’t forget the .rb file extension).
  • Finally, within this file, we define the class:

     class GameController < ApplicationController
  • We inherit from ApplicationController, which in turn inherits from ActionController::Base; much like our models inherited from ActiveRecord::Base via ApplicationRecord.

    Our models inherited .save, .where, and a bunch of other awesome database-related methods from ActiveRecord::Base; whereas our controllers are going to inherit a bunch of methods like render, redirect_to, and a bunch of other awesome interface-related methods from ActionController::Base.

  • Don’t forget the end that goes with the class; type it before you forget it.
  • Move your play_rock action over from application_controller.rb into this new class.
  • Now, when a user visits the path /rock, the “uninitialized constant” error should go away and you should see a response as before.

    If you still see the “unitialized constant” error, then:

    • You named your class wrong; it must exactly match the value in routes.rb, followed by Controller (singular), and CamelCase.
    • You named the file wrong. Try doing .underscore on a string containing the class name in rails console to figure out the correct filename.
    • You put the file in the wrong folder. It has to be within app/controllers/. Not within, for example, app/ or app/controllers/concerns/.
    • You forgot the .rb file extension.
    • If you can’t find which of the above it is, try deleting what you did and paving over your work again from scratch. Sometimes you just can’t spot your own typos, and paving over is the best approach.

You can make as many controllers as you like; in general, a rule of thumb is to have one controller per database table.

For fun

Some day (after you’ve finished all other parts of this week’s homework), make your game pretty:

This is entirely styled using the Bootstrap external stylesheet.

  1. Later we’ll use other methods, post(), etc, if we want to support requests using the other HTTP verbs.