CODEWITHSUNDEEP
javahibernatejakarta-eejpaorm
Daniel Perník
Daniel Perník

Reputation: 5852

JPA Many to many association entity

I am developing Java EE project with embedded database using JPA ORM. My question is...

When I create @ManyToMany fields in two Entities, I have to create association entity too, or it will do container for me?

Course entity snippet

        Public class Course implements Serializable {
    ...
        @Id
        @GeneratedValue(strategy=GenerationType.IDENTITY)
        private long id;

            @ManyToMany
        @JoinTable(name="course_user", joinColumns={@JoinColumn(name="course_id", referencedColumnName="id")},
            inverseJoinColumns={@JoinColumn(name="user_id", referencedColumnName="id")})
        private List<User> enrolledStudents;
    ...

User Entity snippet

public class User implements Serializable {
    private static final long serialVersionUID = 1L;
...

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private long id;

    @ManyToMany(mappedBy="enrolledStudents")
    private List<Course> enrolledCourses;
...

So is this everything I need or not? Thank you for answers!

Upvotes: 0

Views: 173

Answers (3)

Alan Hay
Alan Hay

Reputation: 23226

That will work but see here:

Hibernate Best Practices: Avoiding Many-To-Many and 'exotic' relationships

At some point you will most likely want to save additional info about this relationship (enrollment date for example?) So you may as well create the 'join entity' up front to save refactoring later.

Upvotes: 1

Shenny
Shenny

Reputation: 1

Yes, this is sufficient. Look at examples at http://en.wikibooks.org/wiki/Java_Persistence/ManyToMany

Upvotes: 0

bruno veizaj
bruno veizaj

Reputation: 1

Yes its all you need... if you create the database from the code, you will see three tables... Course, User and Course_User witch has the IDs of Couse and User

Upvotes: 0

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