ruby on rails - How to update my dropdown list without reloading the page -

i've got dropdown list user can select option. functionality works should page reloads each time. i've done bunch of reading , seen examples i'm having hard time synthesizing methods i've seen. know have pass ajax on format.js call in controller method like:

def update_device_position ... if @cabinet.add_cabinet_item(@position_str,params[:position].to_i)   format.js 

then in view, have add form_tag code this:

            <div id="add_device">                 <%= form_tag(:url => {:action => url_for(:controller => "cabinets", :action => :update_device_position), :position => , :id => @cabinet.id }, remote: true)%>                 <td>                     <%= select_tag :position_name, options_for_select(@selection_list) %>                 </td>                 <td>                     <%= hidden_field_tag 'position', %>                     <%= submit_tag "add" , :class => "btn" %>                 </td>              </div>    

when select new value cabinet slot , click submit, routing error stating no route matches [post] "/cabinets/99999999"

thanks.