gistfile1.rb

<% form_for @person, :url => { :action => "update" } do |person_form| %>
    ...
    <% @person.projects.each do |project| %>
      <% if project.active? %>
        <% person_form.fields_for :projects, project do |project_fields| %>
          Name: <%= project_fields.text_field :name %>
        <% end %>
      <% end %>
    <% end %>
  <% end %>