application.js

$(document).ready(function () {
  $('form').submit(function(e){
    e.preventDefault();
    $.ajax({
      type: 'post',
      data: $(this).serialize(),
      url: '/rolls'
    }).done(function(response){
      $('div #die').replaceWith(response);
    });
  });
});

die.erb

  <div id="die">
    <% if @roll %>
      <img src="/<%= @roll.value %>.png" title="<%= @roll.value %>" alt="the roll">
    <% end %>
  </div>

index.erb

<div class="container">
  <h1>Simplest Possible AJAX</h1>
  <p>This contrived app will simulate a roll of a 6-sided die.</p>

  <form method="post" action="/rolls">
    <input type="submit" value="Roll the Die">
  </form>

  <%= erb :die %>

</div>

index.rb

get '/' do
  erb :index
end


# TODO: convert this route to use AJAX
post '/rolls' do

  p request
  # If the user passes-in a "value", let's use it. Otherwise, we'll generate a random one.
  # See: roll_if_value_is_nil method in the Roll model.
  value = params[:value] ? params[:value].to_i : nil

  @roll = value ? Roll.create({ value: value }) : Roll.create

  erb :die, :layout => false  # HINT: what does this do? what should we do instead?
end