CODEWITHSUNDEEP
javahibernate
Squeez
Squeez

Reputation: 959

JPA/Hibernate fetch

I have 2 classes with OneToMany one-directional relationship. 

 class A {
    @Column(name = "a_id")
    Integer id;
    @OneToMany
    @JoinColumn(name = "a_id")
    private List<B> listOfB;
    //getters and setters
}

class B {
    Integer id;
    String name;
    @Column("a_id")
    Integer aId;
    //getters and setters
}

In database I already have saved instance of B. I need to do:

a.listOfB.add(b);
save(a);

In object b in database I have id and name, but fk is null. I need to update fk, but before it I need to fetch object b; How can I do it?

Only by writing custom method, or Hibernate/JPA have its own method to fetch objects?

Upvotes: 0

Views: 68

Answers (1)

v.ladynev
v.ladynev

Reputation: 19956

Your mapping is not very common. Common way to have @JoinColumn in B and mappedBy = "a" in A. And you can specify cascade. In the simplest case cascade = CascadeType.ALL.

    class A {
        @Column(name = "a_id")
        Integer id;

        @OneToMany(mappedBy = "a", cascade = CascadeType.ALL)
        private List<B> listOfB = new ArrayList<B>;

        public void addB(B b) {
           b.setA(this); 
           listOfB.add(b);
        }
    }

    class B {
        Integer id;
        String name;

        @ManyToOne(fetch = FetchType.LAZY)
        @JoinColumn(name = "a_id")
        private A a;
    }

To solve your issue

save(a);
b.setA(a);
saveOrUpdate(b);

And cascade will help you to do the same by almost your way with a help of addB() method

a.addB(b);
save(a);

Upvotes: 1

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