purchases_controller.rb

def show
  @purchase = Purchase.find(params[:id])
end

show.html.erb

<ul>
  <% @purchase.purchase_items.includes(:item).each do |pi| %>
    <li>Item name:           <%= pi.item.name %></li>
    <li>Quantity purchased:  <%= pi.quantity %></li>
  <% end %>
</ul>