Ruby on Rails - Nested Attributes

_attachment_fields.html.erb

<div class="attachment fields">
  <%= f.label :caption %>
  <%= f.file_field :document %>
  <%= link_to_remove_fields "remove", f %>
</div>

_form.html.erb

<!-- other form code -->
<div class="attachments">
  <%= f.fields_for :attachments do |attachment_form| %>
    <%= render 'attachment_fields', :f => attachment_form %>
  <% end %>
</div>
<%= link_to_add_fields "Attach a file or an image", f, :attachments %><br />
<!-- other form code -->

application.js

function remove_fields(link) {
  $(link).prev("input[type=hidden]").val("1");
  $(link).closest(".fields").hide();
}

function add_fields(link, association, content) {
  var new_id = new Date().getTime();
  var regexp = new RegExp("new_" + association, "g")
  $("div.attachments").append(content.replace(regexp, new_id));
}

application_helper.rb

module ApplicationHelper
  def link_to_remove_fields(name, f)
    f.hidden_field(:_destroy) + link_to_function(name, "remove_fields(this)", :class => "btn remove danger")
  end

  def link_to_add_fields(name, f, association)
    new_object = f.object.class.reflect_on_association(association).klass.new
    fields = f.fields_for(association, new_object, :child_index => "new_#{association}") do |builder|
      render(association.to_s.singularize + "_fields", :f => builder)
    end
    link_to_function(name, "add_fields(this, '#{association}', '#{escape_javascript(fields)}')", :class => "btn")
  end
end

attachment.rb

class Attachment < ActiveRecord::Base
  belongs_to :content
end

content.rb

class Content < ActiveRecord::Base
  has_many :attachments
  accepts_nested_attributes_for :attachments, :allow_destroy => true
end