Book.rb

  has_one :splash_page, :dependent => :destroy

  accepts_nested_attributes_for :splash_page, :allow_destroy => true, reject_if: proc { |attributes|  !SplashPage.new(attributes).valid? }

form.rb

  form do |f|
    f.inputs "Splash page", :id => "splash-page" do
      f.inputs :splash_page, :allow_destroy => true, for: [:splash_page, (f.object.splash_page || SplashPage.new)]  do |s|
      	s.input :active
  	#these fields disappear
        s.input :thumbnail, :as => :file, :hint => link_to(image_tag(s.object.thumbnail.url(:thumb)), s.object.thumbnail.url) unless s.object.new_record?
        s.input :header_image, :as => :file, :hint => link_to(image_tag(s.object.header_image.url(:thumb)), s.object.header_image.url) unless s.object.new_record?
      end
    end
  end

splash_page.rb

	validates_presence_of :button_call_to_action, :unless => Proc.new { |c| !c.active? }