NHibernate - reference without (always) doing SQL join - nhibernate

Question

I've got a question that may reek of code/design stink but I'm going to ask it anyway.
Say I've got the two trivial classes below:
public class PostOffice
{
public virtual string ZipCode {get; set;}
public virtual string StreetAddress {get; set;}
}
public class Person
{
public virtual in ID {get; protected set;}
public virtual string FirstName {get; set;}
public virtual string LastName {get; set;}
public virtual PostOffice ZipCode {get; set;}
}
and I've used FluentNHibernate to set PostOffice.ZipCode as the ID and provided the mapping override below:
model.Override<Person>(p=> p.References( z=> z.ZipCode, "ZipCode"));
Person.ZipCode in the database is just a VARCHAR and I would normally write some SQL like
SELECT *
FROM Person
WHERE ZipCode = '90210'
but if I have an NHibernate session and do something like the following:
session.Query<Person>().Where(p=>p.ZipCode == '90210')
the resulting SQL (pretty-printed) looks like
SELECT p.*
FROM Person p INNER JOIN PostOffice o
ON p.ZipCode = o.ZipCode
WHERE o.ZipCode = '90210'
My question - if entities are lazy-load by default, is it possible to specify some mapping properties such that NHibernate will generate the SQL without the join? The example is trivial but in my actual situation, the table being joined to has millions of rows so I'd like to avoid the join if possible.
Thanks in advance!

Answer 1

If the fields ZipCode and StreetAddress are actually in the Person table then you need to map PostOffice as a Component and not a Reference.
http://wiki.fluentnhibernate.org/Auto_mapping#Components
Edit:
You may be able to map a separate property to just that ZipCode field.
public class Person
{
public virtual in ID {get; protected set;}
public virtual string FirstName {get; set;}
public virtual string LastName {get; set;}
public virtual string ZipCode { get; set; }
public virtual PostOffice PostOffice {get; set;}
}
model.Override<Person>(p=>
p.References( z=> z.PostOffice, "ZipCode")
.Not.Update()
.Not.Insert()
);