如何在Rails中执行AJAX-y条件显示/隐藏表单字段?

I'd like to show or hide a few fields in a form in Rails, depending on the state of a select field in the form. I don't (currently) need a second database lookup for the fields, so I'm trying to keep it all confined to the view. I have the outline sketched out, but I'm having some trouble with the implementation details. I'd like to show or hide a few fields in a form