Spring data : CrudRepository's save method and update

I wanted to know if the {save} method in CrudRepository do an update if it finds already the entry in the database like :

@Repository
public interface ProjectDAO extends CrudRepository<Project, Integer> {}

@Service
public class ProjectServiceImpl {

@Autowired private ProjectDAO pDAO;

public void save(Project p) { pDAO.save(p); } }

So if I call that method on an already registred entry, it'll update it if it finds a changed attribute ?

Thanks.

Solution 1:

I wanted to know if the {save} method in CrudRepository do an update if it finds already the entry in the database

The Spring documentation about it is not precise :

Saves a given entity. Use the returned instance for further operations as the save operation might have changed the entity instance completely.

But as the CrudRepository interface doesn't propose another method with an explicit naming for updating an entity, we may suppose that yes since CRUD is expected to do all CRUD operations (CREATE, READ, UPDATE, DELETE).

This supposition is confirmed by the implementation of the SimpleJpaRepository class which is the default implementation of CrudRepository which shows that both cases are handled by the method :

@Transactional
public <S extends T> S save(S entity) {

    if (entityInformation.isNew(entity)) {
        em.persist(entity);
        return entity;
    } else {
        return em.merge(entity);
    }
}

So if I call that method on an already registered entry, it'll update it if it finds a changed attribute?

It will do a merge operation in this case. So all fields are updated according to how the merging cascade and read-only option are set.

Solution 2:

Looking at the default implemantation of CrudRepository interface

 /*
     * (non-Javadoc)
     * @see org.springframework.data.repository.CrudRepository#save(java.lang.Object)
     */
    @Transactional
    public <S extends T> S save(S entity) {

        if (entityInformation.isNew(entity)) {
            em.persist(entity);
            return entity;
        } else {
            return em.merge(entity);
        }
    }

Save method manage two situations:

-If the person Id is null (a new entity is created) then save will call persist method => insert query will be executed.

-If the person id is not null then save will call merge: fetch the existing entity from entityManagerFactory(from the 2 level cache if it doesn't exist then it will be fetched from the database) and comparing the detached entity with the managed and finally propagate the changes to the database by calling update query.

Solution 3:

To be precise, the save(obj) method will treat obj as a new record if the id is empty (therefore will do an insert) and will treat obj as an existing record if the id is filled in (therefore will do the merge).

Why is this important?

Let's say the Project object contains an auto-generated id and also a person_id which must be unique. You make a Project object and fill in the person_id but not the id and then try to save. Hibernate will try to insert this record, since the id is empty, but if that person exists in the database already, you will get a duplicate key exception.

How to handle
Either do a findByPersonId(id) to check if the obj is in the db already, and get the id from that if it is found,
Or just try the save and catch the exception in which case you know it's in the db already and you need to get and set the id before saving.