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NHibernate: Mapping class with generic enum property
public class StateMachineMetadata<T> where T: struct
{
    public virtual int ID { get; protected set; }
    public virtual T State { get; set; }
    public virtual DateTime DateChanged { get; set; }
}

public enum CarState
{
    Stopped = 1,
    Moving = 2
}
public enum OrderState
{
    Ordered = 1,
    Delivered = 2
}

StateMachineMetadata<CarState>, StateMachineMetadata<OrderState>

public interface IStateMachineMetadata<T> where T: struct
{
    int ID { get; }
    T State { get; set; }
    DateTime DateChanged { get; set; }
}

public class StateMachineMetadataBase
{
    public virtual int ID { get; protected set; }
    public virtual DateTime DateChanged { get; set; }
}

public class CarStateMachineMetadata
    : StateMachineMetadataBase, IStateMachineMetadata<CarState>
{
    public virtual CarState State { get; set; }
}

ID | State | DateChanged
1  |   1   |  2012-02-13
2  |   1   |  2012-02-14

ID | State | DateChanged | Type
1  |   1   |  2012-02-13    1
2  |   1   |  2012-02-14    2

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
           assembly="MyAssembly"
           namespace="MyNamespace">
    <class name="StateMachineMetadata`1[MyNamespace.CarState]"
           table="StateMachineMetadata_CarState">
        <!-- property to column mappings -->
    </class>

    <class name="StateMachineMetadata`1[MyNamespace.OrderState]"
           table="StateMachineMetadata_OrderState">
        <!-- property to column mappings -->
    </class>
</hibernate-mapping>