Many-to-Many Bidirectional Relationship
Learn how to change the unidirectional many-to-many relationship to a bidirectional relationship.
We'll cover the following...
In a bidirectional relationship, each side has a reference to the other. In our example, the Category class did not have any reference to the Tournament class. Now we will add a reference to the Tournament class so that the relationship can be navigated from both sides. This will have no effect on the underlying database structure. The join table tournament_catogories already has the foreign keys of both the tournament and category tables, and it is possible to write SQL queries to get tournaments associated with a category.
For a many-to-many relationship, we can choose any side to be the owner. The relationship is configured in the owner side using the @JoinTable annotation. On the inverse side we use the mappedBy attribute to specify the name of the field that maps the relationship in the owning side. From the database design point of view, there is no owner of a many-to-many relationship. It would not make any difference to the table structure if we swap the @JoinTable and mappedBy.
We will begin by creating a List of tournaments in the Category class, along with the getter and setter methods.
public class Category {//...private List<Tournament> tournaments = new ArrayList<>();//...public List<Tournament> getTournaments() {return tournaments;}public void setTournaments(List<Tournament> tournaments) {this.tournaments = tournaments;}@Overridepublic String toString() {return "Category [id=" + id + ", name=" + name + ", tournaments=" + tournaments + "]";}}
mappedBy property
On the tournaments field created above, use the @ManyToMany annotation with the mappedBy property. This shows the value that is used to ...