多选框取舍改变触发事件

多选框选择改变触发事件
<form  name="form" action= "/cad_gift_search_by_object" method="get" >

<select name="object" onchange ="document.form.submit();">
<option value=''>复制往届答谢人选</option>      
<%=options_from_collection_for_select(CadGift.find(:all), "id", "name", params[:object].to_i)%>
</select> 

</form>

后台获取选中的id=params[:object]