non_persistent_nested_records__form_example.html.haml.rb

 = form_for(@order, url:order_url, method:'POST') do |order_form|
    = error_messages_for :order
    
    = order_form.text_field :name
    And more parent form fields...
   
    = order_form.fields_for :order_lines, @order.order_lines do |lines_form|
      = lines_form.text_field :item_number
      And more child form fields..