c# NHibernate -访问一个元素's列表抛出错误(未能惰性初始化…)

我使用的是MVC2, NHibernate 3.2.

我的类:

public class NHibernateHelper
{
    private static ISessionFactory _sessionFactory;
    private static ISessionFactory SessionFactory
    {
        get
        {
            if (_sessionFactory == null)
            {
                var configuration = new Configuration();
                configuration.Configure();
                configuration.AddAssembly(typeof(EstoqueEquipamento).Assembly);
                _sessionFactory = configuration.BuildSessionFactory();
            }
            return _sessionFactory;
        }
    }
    public static ISession OpenSession()
    {
        return SessionFactory.OpenSession();
    }
}

我用this来查询元素:

 public Car GetCar(object pk)
        {
            using (ISession session = NHibernateHelper.OpenSession())
            {
                return session.Get<Car>(pk);
            }
        }

Car映射:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernateClasses" namespace="NHibernateClasses.Entities">
    <class name="Car" table="STACK.CAR" lazy="true" dynamic-update="true" dynamic-insert="true" >
        <id name="CarId" column="CAR_ID" type="Int32" unsaved-value="0">
            <generator class="sequence">
                <param name="sequence">SQ_CAR</param>
            </generator>
        </id>
        <bag name="TireList" inverse="true" generic="true" lazy="true" >
            <key>
                <column name="CarId"/>
            </key>
            <one-to-many class="Tire" />
        </bag>


        <property name="Plate" type="String" column="MAH_PLATE" />

    </class>
</hibernate-mapping>

轮胎映射:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernateClasses" namespace="NHibernateClasses.Entities">
    <class name="Tire" table="STACK.Tire" lazy="true" dynamic-update="true" dynamic-insert="true" >
        <id name="TireId" column="Tire_ID" type="Int32" unsaved-value="0">
            <generator class="sequence">
                <param name="sequence">SQ_Tire</param>
            </generator>
        </id>

        <property name="Brand" type="String" column="MAH_BRAND" />

    </class>
</hibernate-mapping>

然后我查询元素:

var car = GetCar(1);//works just fine
var tirelist = car.Tires;//throws error!

第一行没有问题，但最后一行抛出以下错误:

[汽车]

初始化失败惰性初始化role集合:的车。TireList,没有会话或会话已关闭

如果你还需要配置文件，请告诉我。

谢谢