Save Many to many relationships between two models in Spring

I have two models which has many to many relationship. And I created repository, service, and controller layers for each one. Now when I make a POST request to each one of them, it is created, but how can we make a connection between them since there is a third table created that contains PKs of each one of the models. The same goes with GET request, it return an empty List.

Can Anyone explain to me how can I POST and GET in two models that have many to many relationships and make link between them.

Thanks

Solution

Depending on your implementation of JPA you can achieve the relationship just by annotating the model fields. For example here I use Hibernate and java.persitence annotations to immplement the relationship. I have a class Account which has a list of Subjects. Likewise my Subject has a list of Accounts as a field. To map the relationship I use the java.persitence annotations in this way:

Class Account.java

private List<Subject> subjects;

@ManyToMany(cascade = { CascadeType.ALL })
@JoinTable(
    name = "account_subject", 
    joinColumns = { @JoinColumn(name = "account_id") }, 
    inverseJoinColumns = { @JoinColumn(name = "subject_id") }
)
public List<Subject> getSubjects() {
    if(subjects == null) {
        return new LinkedList<Subject>();
    }
    return subjects;
}

Class Subject.java


private List<Account> accounts;

@ManyToMany(mappedBy = "subjects")
public List<Account>  getAccounts() {
    return accounts;
}

This will automaticlly create a join table called account_subject containing two columns account_id and subject_id, and the line cascade = { CascadeType.ALL } will ensure any operation (update, delete etc..) will be performed on both entities.

To save an Account entity using Hibernate you simply have to set the required field, for example :

List<Subject> mySubjects = new List<Subject>();
mySubjects.add(mySubject);
myAccount.setSubjects(mySubjects);

and in you repository class we use the Hibernate method saveOrUpdate() to persist

sessionFactory.getCurrentSession().saveOrUpdate(myAccount);

where sessionFactory is your Hibernate SessionFactory object.