NHibernate,如何将属性映射到子选择

public class ParentEntity
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual IList<ChildEntity> Children { get; set; }
}

public class ChildEntity
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual ParentEntity Parent { get; set; }
}

<class name="ParentEntity" table="parent_table">
  <id name="Id" column="id">
    <generator class="native" />
  </id>
  <property name="Name" column="name" />

  <bag name="Children" table="child_table" inverse="true" cascade="all">
    <key column="parent_id" />
    <one-to-many class="ChildEntity" />
  </bag>
</class>

<class name="ChildEntity" table="child_table">
  <id name="Id" column="id">
    <generator class="native" />
  </id>
  <property name="Name" column="name" />
  <many-to-one name="Parent" column="parent_id" />
</class>