<natural-id mutable="true|false"/>
        <property ... />
        <many-to-one ... />
        ......
</natural-id>

Even though we recommend the use of surrogate keys as primary keys, you should still try to identify natural keys for all entities. A natural key is a property or combination of properties that is unique and non-null. If it is also immutable, even better. Map the properties of the natural key inside the <natural-id> element. Hibernate will generate the necessary unique key and nullability constraints, and your mapping will be more self-documenting.

We strongly recommend that you implement equals() and hashCode() to compare the natural key properties of the entity.

This mapping is not intended for use with entities with natural primary keys.