How do I get a ComboBox to bind to a String when it's normally an Entity?

Let’s say I have an entities like:

public class Order {
    ....
    Salesrep rep;
    String customerCode;
    ....
}

public class Salesrep {
    ....
    String name;
    int salary;
    ....
}

public class Customer {
    ....
    String firstname;
    String lastname;
    String customerCode;
    ....
}

Now, I have a Form that extends FormLayout that I am binding the Order to like so:

editForm.setData(order);    // order is a passed in Item of type Order
binder = new FieldGroup(order);
binder.buildAndBindMemberFields(editForm);

My comboboxes are defined like:

srCombo.setConverter(new SingleSelectConverter<Salesrep>(srCombo));
srCombo.setItemCaptionPropertyId("name");

cuCombo.setConverter(new SingleSelectConverter<Customer>(cuCombo));
cuCombo.setItemCaptionPropertId("customerCode");

Normally, this works well. In my editForm I have two comboboxes. One for the Salesrep JPAContainer and one for the Customer JPAContainer. Notice, however, that there really is no relationship between the Order and the Customer. In the database, there is a constraint that says “customerCode” must be unique. However, customerCode is NOT the ID of Customer and this cannot be changed.

So Vaadin (and by definition, Hibernate) cannot figure out how to tie the String of customerCode from Order to the String of customerCode in Customer. It’s pretty much a natural key association.

How do I get Vaadin to autoset this attribute for the Customer combobox? The Salesrep combobox is automatically set correctly because there is a proper join.

Thanks.