JPA Persist parent and child with one to many relationship

Question

I want to persist parent entity with 20 child entities, my code is below

Parent Class

@OneToMany(mappedBy = "parentId")
private Collection<Child> childCollection;

---

Child Class

@JoinColumn(name = "parent_id", referencedColumnName = "parent_id")
@ManyToOne(optional=false)
private Parent parent;

---

String jsonString = "json string containing parent properties and child  collection" 

ObjectMapper mapper = new ObjectMapper();
Parent parent = mapper.readValue(jsonString, Parent.class);

public void save(Parent parent) {
    Collection<Child> childCollection = new ArrayList<>() ;

    for(Child tha : parent.getChildCollection()) { 
        tha.setParent(parent);
        childCollection.add(tha);
    }

    parent.setChildCollection(childCollection);
    getEntityManager().persist(parent);
 }

So if there are 20 child tables then I have to set parent reference in each of them for that I have to write 20 for loops? Is it feasible? is there any other way or configuration where I can automatically persist parent and child?

Answer 1

Fix your Parent class:

@OneToMany(mappedBy = "parent")

mappedBy property should point to field on other side of relationship. As JavaDoc says:

The field that owns the relationship. Required unless the relationship is unidirectional.

Also you should explicitely persist Child entity in cycle:

for(Child tha : parent.getChildCollection()) { 
    ...
    getEntityManager().persist(tha);
    ...
}

As Alan Hay noticed in comment, you can use cascade facilities and let EntityManager automatically persist all your Child entities:

@OneToMany(mappedBy = "parent", cascade = CascadeType.PERSIST)

More details about cascades (and JPA itself) you can find in Vlad Mihalcea's blog.