How to prevent validation of relationships in model binder?

Question

An example, if I have a class named Order with a field referencing a Customer, and then an Order form with an drop down list (<%= Html.DropDownListFor(e => e.Customer.ID, new SelectList(...)) %>) for setting the Customer the model binder will create an empty Customer with only the ID set. This works fine with NHibernate but when validation is added to some of the fields of the Customer class, the model binder will say these fields are required. How could I prevent the model binder from validating these references?

Thanks!