NHibernate一对多的问题

public class Department
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual ISet<Employee> Employees { get; set; }
}

public class Employee
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual Department Department { get; set; }
}

    <id name="Id">
       <generator class="identity"/>
    </id>
   <property name="Name"/>
    <set name="Employees" cascade="all">
        <key column="DepartmentId"/>
        <one-to-many class="Employee"/>
    </set>
</class><class name="Employee">
    <id name="Id">
       <generator class="identity"/>
    </id>
   <property name="Name"/>
    <many-to-one name="Department" column="DepartmentId"/>
</class>