How to implement this contraint at the mapping file level ?

[Hussein Baghdadi]

Hi.
My domain objects has a Category object.
This Category object has two relationshipts : parentCategory and childCategories (actually, I'm using CeveatEmptor Category class).
But there is a requirment, no two categories have the same name at the same level (same level here means the same categetory parent).
Well, CeveatEmptor Category.hbm.xml has the following :

<property name="name" type="string">
<column name="NAME"
not-null="true"
length="255"
unique-key="UNIQUE_NAME_AT_LEVEL"/>
</property>
<many-to-one name="parentCategory" cascade="none"
outer-join="false"
foreign-key="FK1_PARENT_CATEGORY_ID">
<column name="PARENT_CATEGORY_ID" not-null="false"
unique-key="UNIQUE_NAME_AT_LEVEL"/>

I tried the following :
insert into category values (1, 0, "Computer", null);
insert into category values (2, 0, "Software", 1);
insert into category values (3, 0, "Porgramming", 2);
insert into category values (4, 0, "Databases", 2);
Here, at the forth line I got an exception about breaking FK contraint.
Any help how to implement my requirement (at the mapping level if possible) ? (I'm not SQL guru)
BTW, what unique-key attribute means ?
Thanks.