To make child attributes accessible to your model through a nested forms (Rails 2.3) you'll need to add the "#{child_class}_attributes" to the attr_accessible method in your parent class. If you don't use attr_accessible in your parent model (you would do this to restrict certain attributes to be accessed via a web form) then you should be all set.

Below is an example where User has_one Profile with the favorite_color attribute being set/updated in the nested form.

class User < ActiveRecord::Base
  has_one :profile #child class
  accepts_nested_attributes_for :profile
  attr_accessible :profile_attributes # the format is the child_class followed by the "_attributes"
end

And the form would like this...

<% form_for @current_user do |f| %>
   <% f.fields_for :profile do |profile| %>
     <%= profile.text_field :favorite_color %>
  <% end %>
<% end %>