Nhibernate许多映射抛出异常

Question

public  class Card
{
    public int CardId { get; set; }

    public string DesignType { get; set; }

    public int Size { get; set; }

}
public class Employee
{
    public int EmpId { get; set; }

    public string Name { get; set; }

    public IList<Card> lstOfCards { get; set; }

}

Mappers：CARD hbm.xml

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="ORLibrary" namespace="ORLibrary.Domain" >

<class name="Card" table="dbo.Cards" lazy="false">

<id name="CardId" type="int">

</id>

<property name="DesignType" />
<property name="Size" />

对于员工：   

<id name="EmpId" type="int"> </id>

<property name="Name" />

<bag name="lstOfCards" table="Emp_Cards" >
  <key column="EmpId" />
  <many-to-many class="Domain.Card" column="CardId"  />
</bag>

现在每次我通过opensession（）创建一个Isession对象引用时，它会抛出异常An association from the table Emp_Cards refers to an unmapped class: Domain.Card