In this exercise you will setup an web application server using Sinatra. In the end, your application will be able to respond to different requests. Please use Getting Started as an additonal source of information for solving the following tasks:

1. Create a new directory for this exercise (for example, use the name “sinatra-exercise”). Create a Gemfile. Add these lines to the Gemfile:

   source 'https://rubygems.org'
   gem 'sinatra'
   

   Then run bundle install in the directory to install sinatra.

2. Next we will create a very basic web application using Sinatra as a starting point. Create a new file called app.rb and add the following to it:

    require 'sinatra'
   
    get '/' do
      'Hello world!'
    end
   

3. Start the server by running ruby app.rb in your terminal. The program will start and continue running until you explicitly stop it.

   It should look similar to this:

   You can now use your browser to make a request to this web server running in your command line. To do so, enter the URL http://localhost:4567. Here’s what you should see:

   

   IMPORTANT: From now on, whenever you make changes in app.rb, you will have to stop and restart your Sinatra application to see the changes. You can stop it with Ctrl + C and start by running ruby app.rb again.

4. Let’s add another route to our application:

    get '/curriculum' do
      'This is our curriculum'
    end
   

   You can visit http://localhost:4567/curriculum to see the result. If you get an error message, then maybe you forgot to restart the app?

5. Remember ERB? We can use it to generate web pages with Sinatra! Try changing the curriculum route so it uses an ERB template:

    get '/curriculum' do
      erb :curriculum
    end
   

   Check the new results in your browser by refreshing the page.

   

   Oh no! We get an error message! However, the message tells us exactly what is wrong: Sinatra is expecting the file views/curriculum.erb to exist in our directory.

6. The :curriculum symbol refers to the name of the template file (curriculum.erb). Sinatra always expects ERB templates in the view directory, hence the path views/curriculum.erb. So let’s create that file and fix the problem!

   Use a <h1> element for the title of our curriculum page. Here’s what it should look like:

   

7. So far, this is a bit boring. Our website is just displaying a static, never changing text. Let’s make it show the current date and time as well. To do this, we can use instance variables (those are the ones with the @). Whenever we assign such a variable in app.rb, we can then use it in the ERB template that gets processed. Add @current_time = Time.now before the erb :curriculum line. Then, in the template, you can display the variable’s value like this: <%= @current_time >.

   Here’s what it looks like:

   

   Note: Whenever you refresh the browser window, the time gets updated, because Ruby code in app.rb runs again and the ERB template gets processed!

8. Next, we want to show a list of Ruby Monstas lessons. For this, we prepared a CSV file (remember those? ;)). You can download it here. Have a look at the file in your text editor. It has just 3 columns: number, lesson and description. Figure out how to parse the CSV file (you have done this before, you can do it again!). You can try this first in irb. Try using the CSV class from the Ruby standard library and see if you can get an array with an entry for each of the rows in the CSV file.

9. Next, show the number and lesson information in the application like in the following screen shot. You can use an instance variable to transfer information from app.rb to the view. Assign a @curriculum variable in app.rb and use its value in the view by looping over the contents.

   

10. Now we are going to create a sub-page for each one of these lessons, where we will show the description from the CSV. First we will make the sub-pages work, then we will create links to get to them. If you enter the URL http://localhost:4567/curriculum/2, we want to see the information for lesson 2, like in the following screenshot. This should also work for all the other lesson numbers, for example http://localhost:4567/curriculum/3, and so on. To do this, we’re going to use the params variable, which is available in app.rb. Take this example from Sinatra’s Getting Started:

    get '/hello/:name' do
      # matches "GET /hello/foo" and "GET /hello/bar"
      # params['name'] is 'foo' or 'bar'
      "Hello #{params['name']}!"
    end
    

    You can use the value in the params variable to find the right entry from your parsed CSV and store that in an instance variable. Then use the instance variable in the ERB template to show the lesson’s information.

    

11. The last thing we want to do is make these sub-pages reachable through the main page. For this, we just have to change the lesson name in the /curriculum page into a link, which goes to the corresponding detail page:

    

12. Congratulations! You wrote your very first web application. Next lesson, we will learn about HTML forms and how to use those to change the data in your web app.