An Overview of Mapping - Beginning Hibernate: From Novice to Professional - page 88

Table 5-10. A Simple Email Table

ID Email

1 [e-mail]

2 [e-mail]

3 [e-mail]

4 [e-mail]

Table 5-11. A Link Table Joining User and Email in a One-to-Many Relationship
UserID

EmailID

1 1

1 2

2 3

2 4

Additional columns can be added to the link table to maintain information on the order-
ing of the entities in the association.
A unique constraint must be applied to the “one” side of the relationship (the userId col-
umn of the UserEmailLink table in Figure 5-7); otherwise, the link table can represent the set of
all possible relationships between User and Email entities—a many-to-many set association.

Figure 5-7. A relationship represented by a link table (duplicates are not permitted because of the
use of a compound primary key)

The Many-to-Many Association
As noted at the end of the previous section, if a unique constraint is not applied to the “one”
end of the relationship when using a link table, it becomes a limited sort of many-to-many
relationship. All of the possible combinations of User and Email can be represented, but it is
not possible for the same User to have the same e-mail address entity associated twice,
because that would require the compound primary key to be duplicated.
If instead of using the foreign keys together as a compound primary key, we give the link
table its own primary key (usually a surrogate key), the association between the two entities
can be transformed into a full many-to-many relationship, as shown in Table 5-12.