I have an Address object. I would like to use this object to represent addresses in many different contexts. E.g. I would like to use to represent a customer's address, an order's address, a building's address, etc. However, since I have to make the association bi-directional, I'm afraid I'm going to have to subclass Address for each use. I.e. CustomerAddress: Address, OrderAddress: Address, etc. This is because each of the addresses need to contain a field which represents it's parent. I.e. CustomerAddress has to have a _customer property. Is there anyway of doing this without doing the subclassing? Or am I stuck doing the subclassing because of NHibernates bi-directionality requirement?

Do you really need the association to be bidirectional (because its not a NH requirement)? If not, the solution is quite simple: don't let the Address table hold the foreign key to your Order, Customer tables, instead have each Customer, Company, Person, Order have a foregin key to your Address table.

The things become really complex if you need a bidirectional association: you will derive all your Company, Order, Person objects from a base class , let say ObjectWithAddress which defines the Address property. This case is covered in the Hibernate documentation when it discusses implicit polymorphism "8.1. The Three Strategies". Check the table per concrete subclass strategy.

Cheers,
Radu