```ruby
module InvoicesHelper
  def display_purchase(invoice)
    unless invoice.purchase.nil?
      invoice.purchase.description
    else
      "(no Purchase set)"
    end
  end
end
```

Add link in Display Purchase Helper
user
```ruby
module InvoicesHelper
  def display_purchase(invoice)
    unless invoice.purchase.nil?
      link_to invoice.purchase.description, invoice.purchase
    else
      "(no Purchase set)"
    end
  end
end
```
assistant