如何使composed_of值对象在字段更改时更新或重新创建自己?

I'm working on a rails-2.3 app that's using composed_of to create an Address class which knows how to geocode itself, among other things. This is all well and good, but given a loaded model that has an address, when I change the fields of which the address is composed on the model, the address instance neither changes nor re-creates itself automatically, and I'm afraid I can't figure out the cleanest way to forcibly make that happen.I'm working on a rails-2.3 app that's using com