Hibernate save object (one to many relationship) foreign key is null

Question

I have one to many relationships between person class and car class. A person can own many cars and vice versa. I am using restful API to post data. My annotations and Get service is working fine but my post service throws " java.sql.SQLIntegrityConstraintViolationException: ORA-01400: cannot insert NULL" error whenever I try to insert new data. Child table foreign key is being inserted as null.

Here is part of my code.

Person.java

 private List<Car> cars = new ArrayList<Car>();

@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.LAZY, mappedBy="person")    
@JsonManagedReference
public List<Car> getCars() {
  return cars;
 }

Car.java

private Person person;  

    @ManyToOne
    @JoinColumn(name = "PERSON_ID", nullable = false, updatable = true, insertable = true)
    @JsonBackReference
    public Person getPerson() {
        return person;
    }

My service class:

@POST
        @Path("/PersonRegistration")
        @Consumes(MediaType.APPLICATION_JSON)
        public Response postPersonCars(Person person) throws Exception{

            Session session = null;     
            ObjectMapper mapper = new ObjectMapper();   
            //Person per = new Person();
            //Car cars = new Car();
            try{
                session = HibernateUtil.getSessionFactory().openSession();
                session.beginTransaction(); 
                //per.setCars(person.getCars());
                session.save(person);
                session.getTransaction().commit();

            }catch(Exception e){
                e.printStackTrace();
                throw e;
            }finally{
                if(null != session){
                    session.close();
                }
            }

            return Response.status(201).entity(mapper.writeValueAsString(person)).build();
        }

Answer 1

This annotation:

@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.LAZY, mappedBy="person")

has two consequences:

1. mappedBy implies that Car is the owning side of the relationship. This means that whenever you want to establish a relationship between Car and Person, you need to do it by setting the Car.person property to the appropriate value. Changes to Person.cars will be ignored by Hibernate.
2. cascade=CascadeType.ALL means that whenever you save a Person, Hibernate will also invoke the save operation on all entities contained in Person.cars

Result: you are calling Session.save() on a bunch of Car entities that do not have the Car.person property set properly.

Solution: either change the owning side of the relationship (be aware that you will also need a @JoinColumn on Person.cars if you do not want an extra database table to be created) or loop through Person.cars and set the Car.person property properly in each of them.

cascade=CascadeType.ALL suggests the first solution fits your use case better.