Conditionally display an image instead of the file input in an ActiveAdmin has_many association.

film.rb

# active_admin and formtastic seem kinda picky about displaying both the
# image and the file input together here... in fact, it seems like this
# is the ONLY way to do this? Adding anything else after the image_tag squashes it.

ActiveAdmin.register Film do
  form do |f|
    f.inputs "Film" do
      f.input :title
    end
    f.has_many :stills do |film_still_form|
      if film_still_form.object.new_record?
        film_still_form.input :photo, :as => :file
      else
        image_tag film_still_form.object.photo.url(:thumbnail)
      end
    end
    f.buttons
  end
end

ActiveAdmin.register Still do
  belongs_to :film
end