JPA entities - EJB 3 in Action - page 372

What gets interesting is if you answer yes to both of these questions. If so, you'll want to
read more about the @ManyToMany relationship in the next section.
9.4.3. Many-to-many relationships
While not as common as one-to-many, many-to-many relationships occur quite frequently
in Enterprise applications. In this type of relationship, both sides might have multiple ref-
erences to related entities. In the ActionBazaar example, the relationship between Cat-
egory and Item is many-to-many, as shown in figure 9.8 .
Figure 9.8. A many-to-many relationship between Item and Category
In the Item-Category relationship, an Item can be placed on multiple categories, and
at the same time a Category can contain multiple Item s. With JPA, the @ManyToMany
annotation is used to configure this relationship. Typically, many-to-many relationships are
bidirectional. Listing 9.20 shows an example of a bidirectional many-to-many relationship
between Item and Category .