如何将下拉列表的默认值设置为相关下拉列表的默认值

请帮助以下人员：

我有3个相关表格

name，network_id，application_id）
网络（id，name，application_id）
应用程序（id，name）

Users (id, name, network_id, application_id) Networks (id, name, application_id) Applications (id, name)

（来自app_id和name为字段的应用程序列表）。用户可以选择网络以及他们希望使用的应用程序（从应用程序列表）。

A network will have a default application selected (from the list of applications with app_id and name as fields). A user can select the network as well as an application they wish to use (from list of applications).

我已经创建了模型，控制器和视图，并且所有下拉列表看起来都很好，并允许我选择一个值BUT如何设置用户的默认值>应用程序下拉到相应的网络> application_id？

I have created the models, controllers and views and and all the dropdowns appear fine and allow me to select a value BUT how would I set the default value of the Users>application dropdown to that of the corresponding Networks>application_id?

的帮助。