drop down list box button

Is is necessary to have a button here?

   <b>Repuestos</b><br />
  <select name="repair_ticket[rp_item]">
   <% @rp_parts.each do |rp_part| %>
       <option value="<%= rp_part.id %>"
         <%= ' selected' if rp_part.id == @repair_ticket.rp_item %>>
         <%= rp_part.name%>
   <% end %>

I want to capture @repair_ticket.rp_item without a button.

just add the onchange="this.form.submit();" like you would normally,
since you aren't using the form helpers properly to build the select
box. It will have to be within a <form> tag though
You can add your ajax to the onsubmit handler of the form tag.

Seriously, though, this is not good rails development, please check
out the "best practice" ways to do things like this before hacking
your own solutions. It's not PHP!