mapping list of polymorphic classes

Category is a self-referencing class with a list<Category> under it. however, there is a subclass LeafCategory extends Category, which could be an item in the list.

for mapping, I have

<class name="model.Category"  table="Category">
            <id name="objectId" type="int" column="OBJECTID">
                  <generator class="hilo">
                        <param name="table">hi_value</param>
                        <param name="column">next_value</param>
                        <param name="max_lo">100</param>
                  </generator>
            </id>
            <property name="name" column="NAME"  not-null="true" />
            <property name="ownerId" column="OWNERID"  />
            <many-to-one name="parentCategory"      class="model.Category" column="PARENTCATEGORYID" not-null="true" insert="false" update="false"  />
            <list name="categories" cascade="all-delete-orphan">
                  <key column="PARENTCATEGORYID" not-null="true" />
                  <list-index column="categoriesIdx"/>
                   <one-to-many class="model.Category"/>
            </list>

            <union-subclass name="model.LeafCategory" table="LeafCategory">

            </union-subclass>

      </class>

I find that I am able to add new Category in its parent's list. but when adding a LeafCategory to the list, I get a
org.hibernate.StaleStateException: Batch update returned unexpected row count from update [0]; actual row count: 0; expected: 1

I have successfully implemented both self-referencing class, and polymorphic classes. it is only when I mix the two into one, that I get this error.

any experts see what is wrong with my mapping?Start Free Trial