Sinatra Forms Params Readme Walkthrough


In this codealong lesson, we'll create a new HTML form and connect it to a Sinatra application so that we can use and manipulate the data provided by the user.


  1. Build a basic HTML form
  2. Connect an HTML form to a Sinatra application via the method and action attributes
  3. Build a POST route in Sinatra's application controller to accept data from a form via params.
  4. Define params as a Ruby hash containing form data.

Green Eggs and Ham

Forms, Forms, Forms!

Think about how many forms you fill out online every day. Credit card payments, logins, registration forms, and even Google searches are all examples of forms. That's because forms are the most common way for users to pass data to a web application.

Specifically in this codealong, we'll connect HTML forms to a Sinatra application by building a form that takes a user's name and favorite food and returns an interpolated string. For example, if the name were "Sam" whose favorite food is "Green Eggs and Ham," we will get an interpolated string of "My name is Sam, and I love Green Eggs and Ham."


Fork and clone this lesson. There are tests to make sure you're on track.

Let's take a quick tour of the starter code. Open app.rb. The only route in our application responds to a get request to the /food_form URL by rendering the HTML in food.erb. We'll be working with app.rb (also called our Application Controller), and food_form.erb in the views directory.

Starting Your Application

To start this Ruby web application, type shotgun in your terminal within the root of this lesson's directory. Your app will be accessible at localhost:9393 in your web browser.

The Favorite Foods Form

Once your shotgun server is up and running, navigate to the /food_form route, localhost:9393/food_form.

You'll see a very basic HTML page. Your first task is to build a form on this page.

Form Review

We're not going to dive too deep into form-making here, but the basics are:

  • HTML forms need a <form> opening tag and a </form> closing tag.

  • Each form field (text, date, password, etc) has a tag (usually <input>) with an attribute type denoting the type of form field. For example, a text box field looks like this:

<input type="text" />
  • A form needs a submit button:
<input type="submit" />

That's all a basic form needs!

Our First Form

Let's build a form with two input fields: one for your name and one for your favorite food. We'll enclose our <input> tags in <p> tags so we can give them some textual context.

Put it all together and your HTML form will look like this:

Copy the code below to views/food_form.erb:

  <p>Your Name: <input type="text" /></p>
  <p>Your Favorite Food: <input type="text" /></p>
  <input type="submit" />

Now if you run shotgun and go to the corresponding view (at the /food_form route in your browser), you'll see your very basic form.

Connecting the Form to your Sinatra App

If you try submitting the form, nothing will happen. That's because the form is not yet connected to our Application Controller in app.rb. There is nothing telling the form to send the user's data to our application.

In order to connect the form to our application, we need to give it explicit directions on where and how to send the data from the user. Both of these pieces of data are attributes that we give our <form> tag.

<form method="POST" action="/food"></form>
  • The method attribute tells the form what kind of request should be fired to the server when the submit button is clicked. In general, forms use a POST request, because it is 'posting' data to the server.

  • The action attribute tells the form what specific route the post request should be sent to. In this case, we're posting to a route called /food

Each form field <input> also must define a name attribute. The name attribute of an <input> defines how our application will identify each <input> data.

Update the form in views/food_form.erb to:

<form method="POST" action="/food">
  <p>Your Name: <input type="text" name="name" /></p>
  <p>Your Favorite Food: <input type="text" name="favorite_food" /></p>
  <input type="submit" />

Let's see what happens when we submit this form.

Sinatra Error

We get a Sinatra error! This is great news. Sinatra errors tell us exactly what we need to do next to make the form work.

Post Routes and Params

The error message Sinatra gives us is telling us that we don't have a route to receive the data from the HTML form that we created in food_form.erb.

If you recall, we gave our form a method attribute of POST and an action attribute of "/food". Again, this is the how and where the data goes from this form.

Every form also needs a corresponding route in the controller file (app.rb). Instead of a get route (which we used to route users to view an html page), we'll set up a post route:

post '/food' do


Notice that both of the attributes from the form are covered in this route: The method post and the action /food. It's almost like a game of catch - the form is throwing the data to the server, which catches it by having the same receiving address (/food) and way of receiving the data (post).

All user submitted data will appear in a params hash accessible throughout our Sinatra controllers. The name attribute of an <input> corresponds to a key in the params hash for that data.

If you create a text field input with <input type="text" name="favorite_food">, whenever the user submits that form, you will be able to access the data entered into the Favorite Foods text box via params[:favorite_food].

This is because we will be passing our data in the form of a hash, where the key will be the name of the data, and the value will be the data itself. In this case, we want our hash (which we call params) to look something like this:

params = {
  :name => "Sam",
  :favorite_food => "Green Eggs and Ham"

So our input names will need to be name and favorite_food.

Param I Am

The data from the form comes nicely packaged up in the form of a hash called params. Let's set the return value of the post route to be params.to_s, and see what our form does now...

post '/food' do

When you submit your form, you should now see the contents of params displayed as a hash in your browser like this:

{"name"=>"Sam", "favorite_food"=>"Green Eggs and Ham"}

Great! This means you've been able to successfully get the data from the form into the controller, and can now manipulate it any way you want.

Let's use the key-value pairs in params to return the following phrase, using good-old string interpolation:

"My name is #{params[:name]}, and I love #{params[:favorite_food]}"

Here's the full post route and action in app.rb:

Add this code to app.rb:

post '/food' do
  "My name is #{params[:name]}, and I love #{params[:favorite_food]}."

Submit the form and see what happens! If you've gotten this far, you can successfully connect an HTML form to your Sinatra app, and you know how to use the params hash to access and manipulate data from the user.

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