code for https://github.com/refinery/refinerycms/issues/2454#issuecomment-27305322

gistfile1.rb

# my form:

<%= form_for [refinery, :projects_admin, @project] do |f| -%>
  <%= render '/refinery/admin/error_messages', :object => @project, :include_object_name => true %>
  <%= render '/refinery/admin/locale_picker', :current_locale => Globalize.locale %>

  <div class='field'>
    <%= f.label :name -%>
    <%= f.text_field :name, :class => 'larger widest' -%>
  </div>
  
  <div class='field'>
    <%= f.label :logo -%>
    <%= render '/refinery/admin/image_picker',
               :f => f,
               :field => :logo_id,
               :image => @project.logo,
               :toggle_image_display => false -%>
  </div>
  
  <fieldset>
    <legend>Teaser</legend>
    <ul id="flat-items">
      <%= f.fields_for(:teasers) do |teaser_builder| %>
        <%= render "/refinery/projects/admin/projects/teaser_fields", :f => teaser_builder %>
      <% end %>
      <%= (link_to_add_fields refinery_icon_tag('add.png'), f, :teasers) %>
    </ul>
  </fieldset>

  <%= render '/refinery/admin/form_actions', :f => f,
             :continue_editing => false,
             :delete_title => t('delete', :scope => 'refinery.projects.admin.projects.project'),
             :delete_confirmation => t('message', :scope => 'refinery.admin.delete', :title => @project.title) -%>
<% end -%>

<% content_for :javascripts do -%>
  <script>
    $(document).ready(function(){
      page_options.init(false, '', '');
    });
  </script>
<% end -%>

# my link_to_add_fields helper (called in form line 27)
def link_to_add_fields(name, f, association)
  new_object = f.object.send(association).klass.new
  id = new_object.object_id
  fields = f.fields_for(association, new_object, :child_index => id) do |builder|
    render(association.to_s.singularize + "_fields", :f => builder)
  end
  link_to(name, '#', :class => "data-add-fields", :data => { :id => id, :fields => fields.gsub("\n", "")}, :tooltip => "Add new")
end

# my _teaser_fields.html.erb partial (which gets rendered in form line 25)
<li>
  <div class="data-item-fields">
    <span class="handle">[Sort]</span>
    
    <div class="field">
      <%= f.label :url_type %>
      <%= f.select :url_type, options_for_select(types_for_select, (f.object.is_external? ? "external" : "internal")), {}, :class => "teaser_url_type" %>
    </div>

    <div class="field data-page <%= f.object.is_external? ? "display_none" : "" %>">
      <%= f.label :page %>
      <%= f.select :page_id, all_pages_for_select, { :include_blank => true }, :class => "teaser_page_id" %>
    </div>

    <div class="field data-url <%= f.object.is_internal? ? "display_none" : "" %>">
      <%= f.label :external_url %>
      <%= f.text_field :external_url, :value => f.object.external_url.blank? ? "http://www." : f.object.external_url %>
    </div>
    
    <%= f.hidden_field :position, :class => "currentposition" %>
    
    <div class="data-destroy-field">
      <%= f.hidden_field :_destroy %>
    </div>
    
    <div class='field'>
      <%= f.label :thumbnail_image -%>
      <%= render '/refinery/admin/image_picker',
                  :f => f,
                  :field => :image_id,
                  :image => f.object.image,
                  :toggle_image_display => false -%>
    </div>

    <div class="field">
      <%= f.label :text %>
      <%= f.text_area :text, :rows => 4, :columns => 50 %>
    </div>
    
    <%= link_to refinery_icon_tag('delete.png'), "#", :class => "data-remove-fields", :tooltip => "Delete" %>
  </div>
</li>

# the relevant js
$("form").on("click", ".data-add-fields", function(event) {
  var regexp, time;
  time = new Date().getTime();
  regexp = new RegExp($(this).data("id"), "g");
  $(this).before($(this).data("fields").replace(regexp, time));
  return event.preventDefault();
});