That is totally up to you.

My line of thought was;
"If I want an invoice, I should ask the `InvoiceProxy`."
This was what felt right within the `rconomic` architecture because if I put it on `DebtorProxy`, I would have to fiddle with `EntityProxy#build`.
And also, I think it is more in line with the "Rails-way".

But the e-conomic API has this on Debtor, which is a quite strong indicator it should be in `DebtorProxy`.