app/views/invoices/show.html.erb

Summary

Maintainability
Test Coverage
<div>
  <h2> Invoice </h2>
  <div class="">
    Invoice Number: <%= @invoice.invoice_number %>
  </div>
  <div class="">
    Customer: <%= @invoice.customer.first_name %>
  </div>
</div>
<%= link_to "Customer", customers_path %>