There is no change to the mapping: hibernate will continue operating as normal. The risk is that an expected constraint might be violated due to programming error, or manual changes to the DB, and then hibernate might barf due to (for example) unexpected missing rows.
As a basic example using User and Role classes:
Code:
<class name="User" ...>
<id name="UserID" .../>
...
<set name="UserRoles" ...>
<key column="UserID"/>
<one-to-many class="Role"/>
</set>
</class>
<class name="Role" ...>
<id ...>
...
</class>
That's all you need for unidirectional one-to-many. For bidirectional, add a many-to-one mapping from Role back to User.
From the sounds of the names, you should consider whether or not a many-to-many mapping might be more appropriate. Can one Role be used by more than one User? If Role is something like Guest, Admin, etc. then it probably can be, in which case you probably want a unidirectional many-to-many mapping.