observe_field, with

http://api.rubyonrails.org/classes/ActionView/Helpers/PrototypeHelper.html#M001633

with 那邊的parameter還可以用prototype 去get 其他element的value

<%= observe_field :target_type_target_type_id,
    :update => :target_list,
    :on => "change",
    :url => {:action => :update_target_list},
    :with => "{target_type_id: value, target_cat_id: $('target_type_target_cat_id').value}"%>