Work in progress.

There are many ways to implement automatic field population on some previous choice. You also need to consider the code to present the form for edition. Here we will use the [prototype] javascript library and the NVelocity view engine. The two example below show countries (full list always populated), state (disabled and only enabled when a country is selected) and city (disabled and only enabled when a state is selected).

Using Prototype and Element Observers

Using Prototype and the onchange event