<% form_for(@user) do |f| %>
  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>

  <% f.fields_for :custom, @user.custom do |custom_form| %>
    <% custom_form.object.marshal_dump.each_key do |custom_field| %>
      <p>
        <%= custom_form.label custom_field %><br />
        <%= custom_form.text_field custom_field %>
      </p>
    <% end %>
  <% end %>
  <p>
    <%= f.submit 'Update' %>
  </p>
<% end %>