_form.html.erb

<%= simple_form_for(@project) do |project_form| %>
  <ul id="goals">
    <%= project_form.simple_fields_for :goals do |goal_form| %>
      <%= render 'goal_form', :goal_form => goal_form %>
    <% end %>
  </ul>
<% end %>

_goal_form.html.erb

<% el_id = goal_form.object.new_record?? 'NEW_RECORD' : goal_form.object.object_id %>

<li id="<%= el_id %>">
  <%= goal_form.input :name %>
</li>

project.rb

class Project < ActiveRecord::Base
  has_many :goals, :dependent => :destroy

  accepts_nested_attributes_for :goals, :allow_destroy => true
end

projects.js

function add_goal(html_form) {
  $('#goals').append( html_form.replace(/NEW_RECORD/g, new Date().getTime()) );
}

projects_helper.rb

def add_goal_link(form_builder)
  html = nil
  form_builder.fields_for :goals, Goal.new, :child_index => 'NEW_RECORD' do |goal_form|
    html = render('goal_form', :goal_form => goal_form )
  end

  link_to_function t('interface.add_goal'), "add_goal('#{escape_javascript(html)}')"
end