View of the dynamic drop down example with Rails, Coffeescript and jQuery

knowledges.js.coffee

jQuery ->
  $('#knowledge_parent_subject').change -> 
    selected_id = $('#knowledge_parent_subject option:selected').val()
    $.get "/subjects/#{selected_id}/subcategories"

knowledges_controller.rb

def new
  # Find top subjects 
  @parent_subjects = Subject.find_all_by_parent_id(nil)
  # Find subjects for the first parent subject
  @subjects = Subject.find_all_by_parent_id(@parent_subjects.first.id)
  @knowledge = Knowledge.new()
end

new.html.erb

 <%= simple_form_for @knowledge,:url => professor_knowledges_path, :html => { :class => 'form-horizontal' } do |f| %>
    <div class="control-group select optional">
      <%= label_tag "Subject Type", nil, :class => "select optional control-label"%>
      <div class="controls">
	<%= select_tag "Parent Subject", options_from_collection_for_select(@parent_subjects, "id", "name"), :id => "knowledge_parent_subject" %>
      </div>
    </div>
    <%= f.input :subject, :collection => @subjects, :label => "Subject", :selected => @subjects.first %>
  <%= f.button :submit, t('add_form'),:class => 'btn-primary' %>
<% end %>

subcategories.js.erb

$("#knowledge_subject").html('<%=j options_from_collection_for_select(@subcategories, "id", "name") %>');

subjects_controller.rb

class SubjectsController < ApplicationController

  def subcategories
    respond_to do |format|
      format.js { @subcategories = Subject.find_all_by_parent_id(params[:id]) } 
    end
  end
end