EDIT: My previous answer (see below) was bugging me because it’s not very nice (it still loops through all the other registration_notes needlessly). After reading the API a bit more, the best way to get the behaviour the OP wanted is to replace:
<%= r.fields_for :registration_notes do |n| %>
with:
<%= r.fields_for :registration_notes, @registration.registration_notes.build do |n| %>
fields_for optionally takes a second parameter which is the specific object to pass to the builder (see the API), which is built inline. It’s probably actually better to create and pass the new note in the controller instead of in the form though (just to move the logic out of the view).
Original answer (I was so close):
Just to clarify, you want your edit form to include a new nested registration note (and ignore any other existing ones)? I haven’t tested this, but you should be able to do so by replacing:
<%= r.fields_for :registration_notes do |n| %>
with:
<%= r.fields_for @registration.registration_notes.build do |n| %>
EDIT: Okay, from a quick test of my own that doesn’t work, but instead you can do:
<%= r.fields_for :registration_notes do |n| %>
<%= n.text_area :content if n.object.id.nil? %>
<% end %>
This will only add the text area if the id of the registration note is nil (ie. it hasn’t been saved yet).
Also, I actually tested this first and it does work 😉