C# - Mapping NHibernate Many-to-one Relationships From Abstract Base Classes To Union-subclasses?

I am having a problem mapping a many-to-one relationship from an abstract base class mapping to a concrete union-subclass. Example:

public abstract class Entity
{
public virtual Guid ID {get; set;}
public virtual string Name {get;set;}
public virtual User OwnerUser {get; set;}
}

public class User : Entity
{
public virtual string UserName {get; set;}
}

I have a base abstract class for all of my database objects. I am mapping these classes with the Entity class as the abstract mapping class and the User as a union-subclass. When creating the configuration object, no errors are thrown and the Schema exports just fine. However, the field to the OwnerUser just won't show up in the database for all of the concrete classes. Here is an example of how the mapping looks

<class entity-name="Entity" name="Entity" abstract="true">
<id name="ID" type="guid">
<generator class="guid.comb"/>
</id>
<property name="Name" />
<many-to-one name="OwnerUser" column="ID" entity-name="User" />........

I am also using an Oracle XE instance as the database backend. If this isn't enough information to properly answer the question, let me know and I will add what I can.