Abstract the type. A REPOSITORY “contains” all instances of a specific type, but this does not mean that you need one REPOSITORY for each class. The type could be an abstract superclass of a hierarchy (for example, a TradeOrder could be a BuyOrder or a Sell-Order). The type could be an interface whose implementers are not even hierarchically related. Or it could be a specific concrete class. Keep in mind that you may well face constraints imposed by the lack of such polymorphism in your database technology.
