NHibernate: How can I combine fields from two classes?

<class name="Person">

       <id name="Id" column="person_id" unsaved-value="0">

              <generator class="native"/>

       </id>



       <property name="Name"/>

       <property name="Sex"/>



       <join table="address">

              <key column="address_id"/>

              <property name="Address"/>

              <property name="Zip"/>

              <property name="Country"/>

              <property name="HomePhone"/>

              <property name="BusinessPhone"/>

       </join>

</class>

public class Image {
    public virtual HostingProvider HostingProvider { get; set; } // NHibernate takes care of this
    public virtual string BaseUrl { get { return HostingProvider.BaseUrl; } }
}