Dynamic Select Boxes for Rails 4 (full) @ https://github.com/bmarcot/dynamic-select-boxes

app_assets_javascripts_welcome.js.coffee

# app/assets/javascripts/welcome.js.coffee

$ ->
  $(document).on 'change', '#countries_select', (evt) ->
    $.ajax 'update_cities',
      type: 'GET'
      dataType: 'script'
      data: {
        country_id: $("#countries_select option:selected").val()
      }
      error: (jqXHR, textStatus, errorThrown) ->
        console.log("AJAX Error: #{textStatus}")
      success: (data, textStatus, jqXHR) ->
        console.log("Dynamic country select OK!")

app_controllers_welcome_controller.rb

# app/controllers/welcome/controller.rb

class WelcomeController < ApplicationController
  
  def index
    @countries = Country.all
    @cities = City.where("country_id = ?", Country.first.id)
  end
  
  def show
    @city = City.find_by("id = ?", params[:trip][:city_id])
  end

  def update_cities
    @cities = City.where("country_id = ?", params[:country_id])
    respond_to do |format|
      format.js
    end
  end

end

app_models_city.rb

# app/models/city.rb

class City < ActiveRecord::Base
  belongs_to :country
end

app_views_cities__city.html.erb

<!-- app/views/cities/_city.html.erb -->

<option value="<%= city.id %>"><%= city.name.titleize %></option>

app_views_welcome_index.html.erb

<!-- app/views/welcome/index.html.erb -->

<%= form_for :trip, url: {action: "show"}, html: {method: "get"} do |f| %>
  <%= f.select :country_id, options_for_select(@countries.collect { |country|
    [country.name.titleize, country.id] }, 1), {}, { id: 'countries_select' } %>
  <%= f.select :city_id, options_for_select(@cities.collect { |city|
    [city.name.titleize, city.id] }, 0), {}, { id: 'cities_select' } %>
  <%= f.submit "Go!" %>
<% end %>

app_views_welcome_show.html.erb

<!-- app/views/welcome/show.html.erb -->

<p><%= @city.name.titleize %></p>
<p><%= link_to 'Go Back', welcome_index_path %></p>

app_views_welcome_update_cities.js.coffee

# app/views/welcome/update_cities.js.coffee

$("#cities_select").empty().append("<%= escape_javascript(render(:partial => @cities)) %>")

config_routes.rb

# config/routes.rb

get 'welcome/update_cities', as: 'update_cities'
get 'welcome/show'

db_seeds.rb

# db/seeds.rb

Country.create(name: "france")
Country.create(name: "italy")

City.create(name: "paris", country_id: Country.find_by(name: "france").id)
City.create(name: "nice", country_id: Country.find_by(name: "france").id)
City.create(name: "roma", country_id: Country.find_by(name: "italy").id)
City.create(name: "venezia", country_id: Country.find_by(name: "italy").id)