It would take ages to go through all files now and assess all belongs_to
associations. So I just declare the old default and then we can move on
and apply the new default for the application while these classes still
use the old one. All new models will then use the new default which is
the goal of this excercise and we can refactor old classes when we touch
them anyway.
There are three main components:
1. The invoice model
2. order serializers: serialize the order for the invoice
3. data presenters: the object that will be use to access the order's serialize data
