NHibernate映射问题

数据表（Order）：
CREATE TABLE [dbo].[Order](
[OrderId] [int] IDENTITY(1,1) NOT NULL,
[OrderDate] [datetime] NULL,
[CustomerId] [int] NULL,
 CONSTRAINT [PK_Order] PRIMARY KEY CLUSTERED 
(
[OrderId] ASC
) ON [PRIMARY]
)
实体类(Order.cs)：
namespace NHibernateSimple.Domain.Entities
{
    public class Order
    {
        public virtual int OrderId { get; set; }
        public virtual DateTime OrderDate { get; set; }
        public virtual int CustomerId { get; set; }
    }
}
映射文件(Order.hbm.xml)：
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernateSimple.Domain" namespace="NHibernateSimple.Domain.Entities">
  <class name="Order" table="Order" lazy="true">
    <id name="OrderId" column="OrderId">
      <generator class="identity" />
    </id>
    <property name="OrderDate" column="OrderDate" />
    <property name="CustomerId" column="CustomerId" />
  </class>
</hibernate-mapping>
配置文件<hibernate.fcg.xml>:
<?xml version="1.0" encoding="utf-8"?>
<hibernate-configuration  xmlns="urn:nhibernate-configuration-2.2" >
  <session-factory>
    <property name="connection.driver_class">NHibernate.Driver.SqlClientDriver</property>
    <property name="connection.connection_string">
      server=zhangfq\zhangfq2008;uid=sa;pwd=flzxl9t;database=Study_Data;
    </property>
    <property name="adonet.batch_size">10</property>
    <property name="show_sql">true</property>
    <property name="dialect">NHibernate.Dialect.MsSql2008Dialect</property>
    <property name="use_outer_join">true</property>
    <property name="command_timeout">10</property>
    <property name="query.substitutions">true 1, false 0, yes 'Y', no 'N'</property>
    <property name="proxyfactory.factory_class">
      NHibernate.ByteCode.Castle.ProxyFactoryFactory,
      NHibernate.ByteCode.Castle
    </property>
    <mapping assembly="NHibernateSimple.Domain" />
  </session-factory>
</hibernate-configuration>
程序运行时加载：
 return (new Configuration()).Configure("D:\\Study\\NHibernateSimple\\父子对应\\NHibernateSimple.Data.Test\\hibernate.cfg.xml").BuildSessionFactory();此处出错：TestFixture failed: SetUp : NHibernate.MappingException : Association references unmapped class: NHibernateSimple.Domain.Entities.Order原因在哪？