How to update one instance of a has_many in a form?

Question

I'm trying to update just one instance of a has_many relationship in a form, I'm relying on accepts_nested_attributes_for (which might be the problem).

This works, updating just one response instance on the responses relationship:

<% parent_form.fields_for(:responses) do |ff| %>
  <%= eval("ff.#{question.field_type} :raw_response") if ff.object.answer_id == question.id%>
<% end %>

...but it generates a bunch of unused, unneeded, hidden fields for the unselected responses.

I'm about to write a custom form help, fields_for_one_instance, before that I'd like a sanity check on the above code.

Thanks.