使用级联保存父实体时如何获取子实体 ID

Question

I'm using spring-data-jpa. After adding a child to parent entity, i save the parent to database. I'd like to get the child's id, but I found what I get is null.

I added @GeneratedValue(strategy = GenerationType.IDENTITY) to getId() method, but it didn't work.

here is model:

@Entity
public class Parent {

    private Integer id;
    private List<Child> childList;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public Integer getId() {
        return id;
    }

    @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @JoinColumn(name = "parent_id")
    public List<Child> getChildList() {
        return childList;
    }

    // setters.....

}

@Entity
public class Child {

    private Integer id;
    private String name;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public Integer getId() {
        return id;
    }

    @Cloumn("name")
    public String getName() {
        return name;
    }

}

the parent entity is already in database, so i find it directly, ParentRepository entends JpaReportory
here is my test code:

Parent parent = parentRepository.findById(1);
Child child = new Child();
child.setName("child");
parent.getChildList().add(child);
parentRepository.save(parent);

System.out.println("child's id: " + child.getId());

the output i get is:

child's id: null

the child is saved to database and has id, but the id of entity in memory is still null, how can I get child's id after saving parent? And because the child I create was cited by other object, i need to get id just in this child rather than find a new object from database.

Answer 1

You have to work with the returned value from the save method:

Parent parent = parentRepository.findById(1);
Child child = new Child();
parent.getChildList().add(child);
parent = parentRepository.save(parent); <---------- use returned value with ids set

System.out.println("child's id: " + parent.getChildList().get(0).getId()); <-- access saved child through parent list

Answer 2

As per the code, you have created child Object and not set any value to its element and then trying to get element from newly created object ( child.getId() ) It will alwas be null, unless you assign value from DB to it.

Parent parent = parentRepository.findById(1);
Child child = new Child(); // Empty child object created
parent.getChildList().add(child);
parentRepository.save(parent);

System.out.println("child's id: " + child.getId()); //Referring empty child object

Here what you can do is :
in the 5th line we have assinged dB value to it

Parent parent = parentRepository.findById(1);
Child child = new Child(); // Empty child object created
parent.getChildList().add(child);
parent = parentRepository.save(parent);
child = parent.getChildList().get(0);// assing db value to it( assingning 1st value of `ChildList`)
System.out.println("child's id: " + child.getId()); //now Referring non-empty child object