The foreign key to the collection owner is non-nullable, but the foreign key to other side is nullable. This causes problems for database that don't allow a nullable column in a primary key.

This is reproduced by:

• org.hibernate.test.cascade.MultiPathCascadeTest on DB2 and Sybase (uses hbm.xml);
• org.hibernate.test.immutable.entitywithmutablecollection.inverse tests