When a composite-id is created that makes use of one or more properties inherited from a base class, NHibernate refuses to honour the CompositeId instructions and creates an auto-id instead. An example below:

public class Superclass
{
public int InheritedId { get; set; }
}

public class Subclass : Superclass
{
public int OwnId { get; set; }
}

public class SubclassMap : ClassMapping<Subclass>
{
public SubclassMap()
{
ComposedId(x => {
x.Property(p => p.InheritedId);
x.Property(p => p.OwnId);
});
}
}

Will llink to failing tests and a fix shortly.