public abstract partial class AlertSpecification
{
    private long _alertSpecificationId;
    private string _specificationName;
    private bool _active;
    private int _createdBy;
    private DateTime _createdOn;

    public virtual long AlertSpecificationId
    {
        get { return _alertSpecificationId; }
        set { _alertSpecificationId = value; }
    }

    public virtual string SpecificationName
    {
        get { return _specificationName; }
        set { _specificationName = value; }
    }

    public virtual bool Active
    {
        get { return _active; }
        set { _active = value; }
    }

    public virtual int CreatedBy
    {
        get { return _createdBy; }
        set { _createdBy = value; }
    }

    public virtual DateTime CreatedOn
    {
        get { return _createdOn; }
        set { _createdOn = value; }
    }
}

public partial class ComponentSpecification : AlertSpecification
{
    private string _vehicleType;

    public virtual string VehicleType
    {
        get { return _vehicleType; }
        set { _vehicleType = value; }
    }
}

public partial class ColdVehicleSpecification : AlertSpecification
{
    private double _sigmaThreshold;

    public virtual double SigmaThreshold
    {
        get { return _sigmaThreshold; }
        set { _sigmaThreshold = value; }
    }
}

<?xml version="1.0" encoding="utf-8"?>
    <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"

     namespace="IS.QueryPerformanceTest.Model" assembly="IS.QueryPerformanceTest.Model" >
        <class name="AlertSpecification" abstract="true">
        <cache usage="read-write"/>
       <id name="AlertSpecificationId" type="Int64">
           <generator class="hilo"/>
       </id>
    <property name="SpecificationName" column="Name" />
    <property name="Active" />
    <property name="CreatedBy" />
    <property name="CreatedOn" />
    <union-subclass name="ColdVehicleSpecification"   table="AlertSpecificationColdVehicle">
        <property name="SigmaThreshold" column="CVSigmaThreshold" />
    </union-subclass>
    <union-subclass name="ComponentSpecification" table="AlertSpecificationComponent">
        <property name="VehicleType" column="VehicleType" />
    </union-subclass>
    </class>
</hibernate-mapping>

var repo = new NHComponentSpecificationRepository(ObjectFactory.GetInstance<ISession>

());

var cvRepo = new

NHColdVehicleSpecificationRepository(ObjectFactory.GetInstance<ISession>());

var allComponentSpecs = repo.FindAll();
var allColdVehicleSpecs = cvRepo.FindAll();

public IList<ColdVehicleSpecification> FindAll()
{
    var result = _session.Query<ColdVehicleSpecification>().ToList();

    return result;
}

ComponentSpecification cs = new ComponentSpecification ();
session.Store(cs);
var csId = cs.Id;

// later on,
// crashes with a cast exception
var cvs = session.Get<ColdVehicleSpecification>(csId);