index.html.erb

<table>
  <thead>
    <tr><th>Name</th></tr>
  </thead>
  <tbody id="specifications">
    <% @specifications.each do |spec| %>
      <tr id="spec-<%= spec.id %>"><td><%= spec.name %></td></tr>
    <% end %>
  </tbody>
</table>

<% content_for :head do -%>
  <%= javascript_include_tag 'jquery.js', 'jquery.tablednd_0_5.js' %>
  <script type="text/javascript" charset="utf-8">
    $().ready(function() {
      $('#specifications').tableDnD({
        onDrop: function(table, row) {
          $.ajax({
             type: "POST",
             url: "<%= url_for(:action => 'sort') %>",
             processData: false,
             data: $.tableDnD.serialize() + '&authenticity_token=' + encodeURIComponent('<%= form_authenticity_token if protect_against_forgery? %>'),
             success: function(msg) {
               alert("The specifications have been updated")
             }
           });
        }
      })
    })
  </script>
<% end -%>

specifications_controller.rb

SpecificationsController < ActionController::Base
  def sort
    Specification.all.each do |spec|
      if position = params[:specifications].index(spec.id.to_s)
        spec.update_attribute(:position, position + 1) unless spec.position == position + 1
      end
    end
    render :nothing => true, :status => 200
  end
end