This is my form partial:
<%= f.simple_fields_for :photo_attributes, :html => { :multipart => true } do |d| %>
<%= d.label :image, :label =>
You could write something like
<%- form_url = @object.new_record? ? :photo_attributes : :photo %>
<% f.simple_fields_for form_url, :html => { :multipart => true } do |d| %>
That is, if you have an @object to check against. Otherwise you could use action_name (and even controller_name).
So something like:
<%- form_url = action_name == :edit ? :photo : :photo_attributes %>
<% f.simple_fields_for form_url, :html => { :multipart => true } do |d| %>
Hope this helps.