composite Key and inheritance - nhibernate

Question

i have the following classes and mappings
abstract class BaseClass
{
public virtual int Keypart1 { get; set; }
public virtual int Keypart2 { get; set; }
// overridden Equals() and GetHashCode()
}
class InheritingClass : BaseClass
{
}
class BaseClassMap : ClassMap<BaseClass>
{
public BaseClassMap()
{
CompositeId()
.KeyProperty(x => x.Keypart1)
.KeyProperty(x => x.Keypart2);
}
}
class InheritingClassMap : SubclassMap<InheritingClass>
{
public InheritingClassMap()
{
KeyColumn("Keypart1");
KeyColumn("Keypart2");
}
}
inserting, updating and session.Get() work fine but querying like
var result = session.CreateCriteria<InheritingClass>().List<InheritingClass>();
throws
NHibernate.InstantiationException: Cannot instantiate abstract class or interface: ConsoleApplication1.BaseClass
bei NHibernate.Tuple.PocoInstantiator.Instantiate()
bei NHibernate.Tuple.Component.AbstractComponentTuplizer.Instantiate()
bei NHibernate.Type.ComponentType.Instantiate(EntityMode entityMode)
bei NHibernate.Type.ComponentType.Instantiate(Object parent, ISessionImplementor session)
bei NHibernate.Type.EmbeddedComponentType.Instantiate(Object parent, ISessionImplementor session)
bei NHibernate.Type.ComponentType.ResolveIdentifier(Object value, ISessionImplementor session, Object owner)
bei NHibernate.Type.ComponentType.NullSafeGet(IDataReader rs, String[] names, ISessionImplementor session, Object owner)
bei NHibernate.Loader.Loader.GetKeyFromResultSet(Int32 i, IEntityPersister persister, Object id, IDataReader rs, ISessionImplementor session)
bei NHibernate.Loader.Loader.GetRowFromResultSet(IDataReader resultSet, ISessionImplementor session, QueryParameters queryParameters, LockMode[] lockModeArray, EntityKey optionalObjectKey, IList hydratedObjects, EntityKey[] keys, Boolean returnProxies)
...
it seems NH tries to instantiate the abstract baseclass as a compositekey and fails. Can i somehow work around that?
UPDATE: my testcode
var config = Fluently.Configure()
.Database(SQLiteConfiguration.Standard.InMemory().ShowSql().FormatSql())
.Mappings(m => m.FluentMappings
.Add<BaseClassMap>()
.Add<InheritingClassMap>()
)
.BuildConfiguration();
var sf = config.BuildSessionFactory();
using (var session = sf.OpenSession())
{
new SchemaExport(config).Execute(false, true, false, session.Connection, null);
var obj = new InheritingClass
{
Keypart1 = 1,
Keypart2 = 2,
};
session.Save(obj);
session.Flush();
session.Clear();
// throws here
var result = session.CreateCriteria<InheritingClass>().List<InheritingClass>();
}

Answer 1

How does your database look like? According to your mapping, you are using a table-per-subclass mapping. In this case NHibernate will try to create an instance of BaseClass if no row is found in the table of InheritingClass.
edit: There is a abstract="true" attribute for a <class> in NHibernate mapping which might fix your problem. But it seems like this isn't exposed in Fluent NHibernate for a ClassMap, only for SubclassMap (which won't help you).
But maybe you could also fix the problem by using a component for the composite ID (so that NHibernate doesn't need to create a BaseClass object for its EntityKey. See here for infos about that.

Answer 2

thx to cremor this is what i end up with
abstract class BaseClass
{
public virtual BaseClassId Key { get; set; }
}
class BaseClassId
{
public virtual int Keypart1 { get; set; }
public virtual int Keypart2 { get; set; }
public override bool Equals(object obj)
{
var other = obj as BaseClassId;
return (other != null) && (Keypart1 == other.Keypart1) && (Keypart2 == other.Keypart2);
}
public override int GetHashCode()
{
unchecked
{
return Keypart1 + Keypart2;
}
}
}
// mapping
CompositeId(b => b.Key)
.KeyProperty(x => x.Keypart1)
.KeyProperty(x => x.Keypart2);
var obj = new InheritingClass
{
Key = new BaseClassId
{
Keypart1 = 1,
Keypart2 = 2,
}
};