+ button to add nested object to fields_for & accepts_nested_attributes

_form.html.erb

<%= form_for(@foo) do |f| %>
  <%= f.fields_for :bars do |b| %>
    <%= b.text_field :name %>
  <% end %>

  <%= f.submit "+", :name => "actions[add_bar]" %>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

foos_controller.rb

class FoosController < ApplicationController
  respond_to :html

  def create
    @foo = Foo.new(params[:foo])

    if params[:actions][:add_bar]
      @foo.bars.build
      render "new"
    else
      flash[:notice] = "Foo was successfully created." if @foo.save
      respond_with(@foo)
    end
  end
end