An Overview of Mapping - Beginning Hibernate: From Novice to Professional - page 86

Figure 5-4. A single entity representing a one-to-one relationship

Alternatively, the entities can be maintained in distinct tables with identical primary keys,
or with a key maintained from one of the entities into the other, as in Tables 5-6 and 5-7.

Table 5-6. The User Table

ID Username

1 dcminter

2 jlinwood

3 tjkitchen

Table 5-7. The Email Table

ID Username

1 [e-mail]

2 [e-mail]

3 [e-mail]

It is possible to create a mandatory foreign key relationship from one of the entities to
the other, but this should not be applied in both directions because a circular dependency
would be created. It is also possible to omit the foreign key relationships entirely (as shown
in Figure 5-5) and rely upon Hibernate to manage the key selection and assignment.

Figure 5-5. Entities related by primary keys