Our users place requests by visiting URLs in our app. In order to respond to requests dynamically (as opposed to sending back static, unchanging pages of HTML), we need to route each request to a Ruby method that can do the desired work (reading from an API, picking a random number, CRUDing from the database, etc) and then send back the output (formatted in HTML1).

Our apps are essentially defined by the list of URLs that we allow users to visit, and for which we provide responses. We keep this list of URLs in one very important file: config/routes.rb.

Here’s an example routes.rb:

# /config/routes.rb

Rails.application.routes.draw do
  match("/rock", { :controller => "game", :action => "play_rock", :via => "get" })
  match("/paper", { :controller => "game", :action => "play_paper", :via => "get" })
  match("/scissors", { :controller => "game", :action => "play_scissors", :via => "get" })
  match("/", { :controller => "game", :action => "homepage", :via => "get" })


  • 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 match method and its two arguments:
    • The first argument to match 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 match 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 three 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.
      • :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.
      • :via: The value for this key must be either "post", "get", "patch", or "delete".

        In HTTP (the protocol for exchanging resources over the internet), these are the names for CREATE, READ, UPDATE, and DELETE, respectively; and are known as “HTTP verbs”.

        We use the :via key to specify which CRUD operation best describes the work we’re going to perform in the method. It turns out that most requests involve reading info, rather than creating/updating/deleting it; so we’ll use "get" most often. If you can’t think of exactly which one to use, just default to "get".


Suppose we wrote a route in config/routes.rb that looks like this:

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

Now when a user visits /rock, instead of seeing a “No route matches” error, they will instead see an error uninitialized constant GameController. Progress!

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 a Rails base class called ApplicationController, much like our models inherited from ApplicationRecord. Our models inherited .save, .where, and a bunch of other awesome database-related methods from ApplicationRecord; whereas our controllers are going to inherit a bunch of methods like render, redirect_to, and a bunch of other awesome request/response-related methods from ApplicationController.
  • Don’t forget the end that goes with the class; type it before you forget it.
  • Now, when a user visits the path /rock, the “uninitialized constant” error should go away. Progress!

    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.


The next error the user will encounter is:

The action 'play_rock' could not be found for GameController

This is good! That means we successfully defined GameController. Now we have to define a method within it called play_rock, since that’s what we specified as the value for the :action key back in our route. Let’s do it:

class GameController < ApplicationController
  def play_rock

The primary job of an action is to send back a response to the user — either some HTML for their browser to render, or perhaps redirecting them to some other URL. 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 GameController < ApplicationController
  def play_rock

As you can see, the argument to redirect_to is a string which contains some absolute 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 back to an index page of photos after they’ve deleted a photo.


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

class GameController < ApplicationController
  def play_rock

The argument to render is a string that contains the name of a folder and a file that we’re going to place some HTML in, which we want to be sent to the user’s browser when they visit /rock.

  • We can choose any folder and file name that we want — I just made up the folder name of game_templates and the file name of user_plays_rock. These files are known as our view templates.
  • The extension of the file should be .html.erb. The .erb stands for embedded Ruby, and this is what is going to allow us to make our HTML dynamic and awesome instead of static and boring.


If we’ve defined our action correctly, the next error a user sees when they try to visit /rock is:

Missing template game_templates/user_plays_rock.html.erb with {:locale=>[:en], :formats=>[:html], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby, :arb, :coffee, :jbuilder]}. Searched in:
  * "/home/ubuntu/workspace/app/views"

We’re almost there! We just have to create this file. Notice from the error message that Rails searched within the folder app/views/. This is where we’ll store all of our view templates. Let’s create the folder and file we decided upon, app/views/game_templates/user_plays_rock.html.erb.

Within that file, you know what to do:

<p>Hello, world!</p>

Now, finally, when a user visits /rock, they should see a page and no more errors. We’ve connected the Route-Controller-Action-View (RCAV) dots successfully!


The point of all of this work, as opposed to just creating a file called public/rock.html, is that we can now embed Ruby inside our HTML:

<p>You played rock!</p>

<p>The computer played <%= ["rock", "paper", "scissors"].sample %>!</p>

At last — our game is finally dynamic! The <%= %> (embedded ruby tags, or ERB tags) allow us to write any Ruby expression we want and it will be evaluated, converted into a string, and injected into the source code of the HTML document before the HTML is sent to the browser. The browser will never know that Ruby was involved (you can verify this by Viewing Source of the final page in your live application preview).

There’s also another version of the ERB tag, <% %> (without the equal sign) that does not inject the value into the source of the HTML; this is useful for doing some computation or control flow with conditionals:

<p>You played rock!</p>

<% computer_move = ["rock", "paper", "scissors"].sample %>

<p>The computer played <%= computer_move %>!</p>

<% if computer_move == "rock" %>
  <p>You tied!</p>
<% elsif computer_move == "paper" %>
  <p>You lost!</p>
<% else %>
  <p>You won!</p>
<% end %>
  1. The format of the response could also be JSON, PDF, CSV, XML, or others — and we can offer multiple formats at once. If we wanted to add an iPhone or Android app, for example, we would add a second format — JSON — and then let the native app read our API.