Hibernate version: 2.1.6

I am trying to generate a table that has a unique constraint on the dicriminator column and one other column. No matter what I do, the table is not generated with the discriminator column included in the unique constraint. Am I doing something wrong or is this even possible?

Mapping documents:
...
<discriminator
column="regStepType"
type="string"
>
<column
name="regStepType"
unique-key="one"
/>
</discriminator>

...

<many-to-one
name="userGroup"
class="com.wingateweb.vo.UserGroup"
cascade="none"
outer-join="auto"
update="true"
insert="true"
access="property"
>
<column
name="usergroup_id"
unique-key="one"
/>
</many-to-one>

Name and version of the database you are using: HSQL 1.7.2.2