Is there a way to use a Join mapping in fluent nhibernate with a filter specified?

I have a scenario where I want to create map a class that pulls data from two table 开发者_运维知识库together. For example, my two tables are Persons and PersonAddresses. I want the resulting mapped class to merge these two together like this:

public class PersonWithAddress
{
   public int ID { get; set; }
   // person fields ..

   // address fields
}

The problem is each person can have more than one address, but only one address is marked as their primary address. That is the address I want to pull into this entity. So I thought there was a way to use Join() in my mapping and specify a filter on the join like this:

public PersonWithAddressMap()
{
   Table("People");
   Id(x => x.ID, "PersonID");

   Map(x => x.FirstName);
   // map all the person fields...

   Join("PersonAddresses", pa =>
   {
      pa.Optional();

      pa.Map(x => x.AddressLine1);
      // map all the other address fields

      // apply filtering
      pa.Where(x => x.IsPrimary == true);  // <-- does not exist
   });
}

Anyone know of a way to do this?