tap/app/views/orders/new.html.erb

<h3>Order for <%= @user.nickname %> (<%= euro(@user.balance) %>)</h3>

<div class="row">
  <%= f_form_for [@user, @order] do |f| %>
    <%= f.error_messages %>

    <div class="col-md-12">
      <%= f.fields_for :order_items do |op_field| %>
        <%= render op_field.object, f: op_field, product: op_field.object.product %>
      <% end %>
    </div>

    <%= render 'order_items/total_price', f: f %>
  <% end %>
</div>