fields_for and one out of many from association

class Person < ActiveRecord::Base
  has_many :addresses
  has_one :primary_address, :class_name => 'Address', :order =>
'preferred desc, id'

  accepts_nested_attributes_for :addresses, :allow_destroy => true
  accepts_nested_attributes_for :primary_address, :allow_destroy =>
true
end

class Address < ActiveRecord::Base
  belongs_to :person
end

In the view:
<% f.fields_for :primary_address do |address_builder| %>
  <%= render 'addresses', :f => address_builder %>
<% end %>

The 'addresses' table has a boolean column named 'preferred' and there
should be only one address for the person that is the preferred one.

Testing on the console the 'has_one' definition gives me the
'preferred' address if there is one, otherwise it gives me the first
one it finds or returns nil if there are no addresses for the person.
All this is working as expected.

If my association has 2 addresses (I'm making sure one of them is
'preferred' and the other one is not), when the view code runs it does
not output just one address (which is what I need), it outputs 2
addresses, actually the same (preferred) address twice.

Is there a way to make fields_for do what I need it to do?

Thanks in advance.

Never mind. I think I got it.