The FAQ says:

I'm using a one-to-many association with a NOT NULL constraint on the key column and Hibernate causes constraint violations.
...
Or, for a unidirectional association, use <key not-null="true" ... /> in the collection mapping.

However, it seems not-null isn't an attribute for key. My program works if I remove the constraint, but I'd rather not do that and keep my dba happy if I can. I'm attempting to put it on the key column under the bag:


<bag name="SomeValues" table="hibernatechilld" cascade="all">
<key not-null="true" column="parent_id" />
<one-to-many class="HibernateTest.someValue, HibernateTest"/>
</bag>

I get an error stating that's not valid. What's the real answer for unidirectional, or is bi-directional a requirement?

A unidirectional one-to-many indeed requires the foreign key column to be nullable.

There are two options to work around that: First, make the association bi-directional. Secondly, if the elements in the collection does not need to be full-fledge entities (i.e. they live and die with the parent object), you may use <composite-element> instead of <one-to-many>.

The first option is arguably easier to implement; the second option requires some careful considerations on the collection types (e.g. bags vs. sets) and equality semantics.