app/assets/javascripts/event.js
function show_subform(){
var v = $('body.edit.events #event_type, body.new.events #event_type').val();
if(!v) return;
var c = v.substr(1+v.lastIndexOf(":"));
$('.subform').hide();
$('.subform.' + c ).show();
}
$(document).ready(function(){
$('body.edit.events #event_type, body.new.events #event_type').on('change', show_subform );
show_subform();
});