JAVA Hibernate merge is updating and not merging

Question

When I use merge I expect that hibernate will copy not null values to a persisted entity and update the database. So in the bellow use-case, I expect that the merge will keep lastName with the value Last_0 and change firstName to the value First_1 .

but actually, hibernate is just updating, meaning that lastName become null after the code execute

    @Transactional
    public void create()  {
        SomeEntity create = new SomeEntity();
        create.setFirstName("First_0");
        create.setLastName("Last_0");
        this.getSession().save(create);
        //now in the database id:1, firstName:First_0, lastName:Last_0
   }


   @Transactional
    public void merge()  {
        SomeEntity merge = new SomeEntity();
        merge.setId(1);
        merge.setFirstName("First_1");
        this.getSession().merge(merge);
         // now in the database id:1, firstName:First_1, lastName:null
    }
    create();
    merge();

I expect lastName to be with the value of Last_0 since it is merging. but it is null.

Thanks

Answer 1

You are confused about how Hibernate's merge() is implemented in practice. merge() is implemented under the hood in this case via a SQL UPDATE , meaning something like the following:

UPDATE some_entity
SET
    firstName = 'First_1',
    lastName = NULL
WHERE
    id = 1;

That is, it will overwrite all the field values with whatever your detached entity happens to have. Note that if no record with this primary key id=1 existed, then Hibernate would have done an INSERT , not an update.

If you wanted to start off with the original values for the entity, then you should have used a get:

SomeEntity merge = (SomeEntity) this.getSession().get(SomeEntity.class, 1);
merge.setFirstName("First_1");
this.getSession().merge(merge);

Now the original values will "stick," except for the first name, which was overwritten to something else.

Answer 2

If you want hibernate to only save part of fields and not all of them use @DynamicUpdate

Example here: https://www.baeldung.com/spring-data-jpa-dynamicupdate

As posted inside:

Actually, when we use @DynamicUpdate on an entity, Hibernate does not use the cached SQL statement for the update. Instead, it will generate a SQL statement each time we update the entity. This generated SQL includes only the changed columns.