Sinatra Dynamic Routes Codealong


In this code-along lesson, we'll learn why dynamic routes are powerful and how to integrate them into a Sinatra project.


  1. Explain the purpose of dynamic routes
  2. Create dynamic routes in the controller
  3. Use URL params to help get the text from the URL into the views
  4. Show the relationship between dynamic routes and the browser URL

Why Dynamic Routes Matter

How does AirBnB create a separate url for every property it hosts on its site? Would it make sense to hard-code hundreds of thousands of routes ( get '/property1', get '/property2',get '/property2356') in the controller to display each rental property? The controller would get messy and long very quickly. Instead, AirBnB (and Twitter, and Facebook, etc) use dynamic routes - routes that are created based on attributes within the url of the request. In this code-along we'll learn why dynamic routes are powerful and how to integrate them into a Sinatra project.

To code along, fork and clone this lab. Run bundle install to make sure all of your dependencies are installed. Run shotgun to make sure that your application can run. There are tests, so make sure you're running learn test periodically to make sure your code is behaving as expected.

Starter Code

Open up app.rb in your text editor. You'll notice two routes, get '/hello' and get '/hello/:name'.

The first route is familiar looking to us. It returns the string "hello world" in the browser when we go to the url. This is an example of static routing, which we've seen.

But get '/hello/:name' is very different. What's with that : in front of name? This is an example of a dynamic route.

Eventually we are going to need to capture data from the user. We need to know who they want to say hello to. There are a few ways to get this information, and the easiest is built right into the URL. They are already typing that URL into the box at the top of their browser, so let's use it to get a bit more information.

Dynamic Routes

In your browser, head to http://localhost:9393/hello/danny. Now go to http://localhost:9393/hello/victoria and http://localhost:9393/hello/lyel. Notice how the content on the page changes depending on what we type as the URL in the browser. This is the beauty of dynamic routing - it allows us to take input straight from the url, instead of through a form. In doing so, we can modify the content of a view at the moment the get request is received by the controller.

How Dynamic Routes Work:

It's important to note that in Sinatra a route is simply an HTTP method/verb that is paired with a URL-matching pattern. When your Sinatra application receives a request, it will match that route to a specific controller action that matches that URL pattern.

The best way to explain routes is by going through an example. Our application is a medicine application that has an array containing three instances of a Medicine class.

Here's our array:

all_the_medicines = [
  #<Medicine:0x007fb739b1af88 @id=1, @name="penicillin" @group="antibiotic">,
  #<Medicine:0x007fb739b1af88 @id=2, @name="advil" @group="anti-inflammatory">,
  #<Medicine:0x007fb739b1af88 @id=3, @name="benadryl" @group="anti-histamine">

Our application gets a request :GET /medicines/1. What happens here?

The first thing Sinatra does is try to match the request to a specific controller action. The controller action it would match is as follows: get '/medicines/:id'. Once the request has been matched to the controller action, it then executes the code inside of the controller action block, as shown below:

# medicines_controller.rb
get '/medicines/:id' do
  @medicine = do |medicine| == params[:id]
  erb :'/medicines/show.html'

Let's run through this specific scenario. The HTTP request verb, GET matches the get method in our controller. The /medicines path in the HTTP request matches the /medicines path in our controller method. Finally, the 1, which is an id parameter that's being passed into the path, matches the controller's expectation for an id parameter to be passed in place of :id.

URL Params

A URL parameter is a variable whose values are set dynamically in a page's URL, and can be accessed by its corresponding controller action. It's a very similar concept to a dynamic url.

The next thing that happens is that the all_the_medicines array is queried for a medicine object that has the id of 1. It seems to match this entry: #<Medicine:0x007fb739b1af88 @id=1, @name="penicillin" @group="antibiotic">,. The attributes from this object are assigned to the variable @medicine.

Finally, the @medicine object is rendered via the show.html.erb template inside of the views/medicines directory.

Going back to our initial example, if you played around enough with the examples above, you'll notice that whatever name you typed in the url also appeared in the browser, saying hello to that person. How were we able to get the text from the URL to the views?

URL params help us get the text from the URL into the views. That :name in the route name is just a symbol that will be filled in with text later. The data is passed from the URL to the controller action through an automatically generated hash called params. Don't worry too much about how the hash is created. Just know that inside your controller action, you automatically have access to this hash through the variable params.

To continue the medicine example, the hash looks something like this:

params = {
  :id => "1" //This is a String and NOT an Integer

Note: Values in params always come in as strings, and your return value for the route should also be a string (use .to_i and .to_s).

The key of the hash is determined by the symbol in the url (:id), and the associated value will be the content in the url provided by the user (1). Once inside the controller action, we can access the value from the params hash, just like we would any other hash.


You can receive multiple pieces of data through a dynamic route by separating the content with a forward slash. For example, get '/addnumbers/:number1/:number2' would give you a params hash with two key-value pairs (number1 and number2).

Create your Own Dynamic Routes

Using the example dynamic route that we included in the code-along as a template, create the following two routes:

  • get '/goodbye/:name, a dynamic route that returns "Goodbye, (person's name).", a string. For example, navigating to localhost:9393/goodbye/jerome should display Goodbye, jerome.

  • A dynamic route starting with /multiply that accepts two params (num1 and num2) and returns the product of the two numbers.

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