NHibernate中使用generator为assigned的问题

Hibernate version:
1.0.2.0
Mapping documents:
Parent.hbm.xml 

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
  <class name="HYLQ.Core.Domain.Parent, HYLQ.Core" table="Parent">
    <id name="Id" column="Id" unsaved-value="0">
      <generator class="assigned" />
    </id>
    <property name="Title" type="String" length="200" />

    <bag name="Childs" lazy="true" table="Child" inverse="true" cascade="all" 
            access="NHibernate.Generics.GenericAccessor, NHibernate.Generics" >
      <key column="pid" />
      <one-to-many class="HYLQ.Core.Domain.Child, HYLQ.Core" />
    </bag>
    
  </class>
</hibernate-mapping>

Child.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
  <class name="HYLQ.Core.Domain.Child, HYLQ.Core" table="Child">
    <id name="Id" column="Id" unsaved-value="0">
      <generator class="assigned" />
    </id>
    <property name="Memo" type="String" length="200" />
    <many-to-one name="Parent" column="pid" class="HYLQ.Core.Domain.Parent, HYLQ.Core"
            access="NHibernate.Generics.GenericAccessor, NHibernate.Generics" />
  </class>
</hibernate-mapping>

测试代码：

using (ISession session = TestCategory.Factory.OpenSession())
            {
                int id = 1;

                Parent parent = new Parent();

                parent.Id = id;
                parent.Title = "tetetet";

                Child child = new Child();
                child.Id = 1;
                child.Memo = "222";
                child.parent = parent;

                parent.Childs.Add(child);

                ITransaction trans = session.BeginTransaction();

                try
                {
                    session.Save(parent);
                    trans.Commit();
                }
                catch
                {
                    trans.Rollback();
                    throw;
                }
            } 

当我设置了
chilid.Id =1的时候，则出现了
Test method TestProject1.TestCategory.AddParentChild threw exception: NHibernate.HibernateException: SQL insert, update or delete failed (expected affected row count: 1, actual affected row count: 0). Possible causes: the row was modified or deleted by another user, or a trigger is reporting misleading row count..
跟踪执行的SQL语句后，发现并不是Insert Child，而是Update Child。
当我将chilid.Id =1注释后，就正常，不过chilid.Id他用 unsaved-value="0"代替。
执行的SQL语句都是正常的。

而我在网上看到一些One2Many的例子中其中表的主键都是自增长的后非assigned的。
所以这个问题很是困惑，最后，自己写了一个自定义的Generator来生产ID,
后来就没有出现上述的情况了。

你们碰到过这种情况吗？