为什么使用observe_field代码不能使用JQuery模型对话框?

I'm developing a project on rails and I got a problem on getting the selected item on drop down menu. It works fine on views/applications but when I try to use the same code to get the selected item at the jQuery model dialog box, it doesn't work for me. Can someone explain the problem in this? Can't I use the observe_field on model dialog box?I'm developing a project on rails and I got a p