[dev] Rdo: string as primary key

[Sebastian Birnbach]

I have an Rdo object of type 'dossier' that is related many-to-one to
another RDO object of type 'customer'. One customer information is
prominently more often sought than the rest and that is a (unique) 2-4
letter code.

Is there a way to use the letter code as primary key for the customer
object and set up the relationship accordingly? This would save me from
fetching the customer object in ~80% of all cases.