One-to-Many Unidirectional Relationships 

The one-to-many unidirectional relationship differs from one-to-many bidirectional relationships by the missing field of persistence capable class type on the many-side.

The relational model is the same as the relational model for one-to-many bidirectional relationships.

The mapping metadata for the remaining field of type java.util.Set is the same as the mapping metadata in the one-to-many bidirectional case, except that the property update in the relationship field must not be set to false.

When using one-to-many unidirectional relationships, you have to make sure that no instance of the many-side is referred by more then one instances of the one-side. If not, the relationship is many-to-many, and there is no way to reflect this in the data store using a single foreign key.