Getting "detached entity passed to persist" while trying to update entity

Question

I'm trying to update an object. As part of the process I load the old one from the database and "transfer" its property values to a new object.

But when I try to persist the new object I get the following error:

org.hibernate.PersistentObjectException: detached entity passed to persist: some.package.domain.Exercise

My entity looks as the following

@Entity
class Exercise(
        val name: String,
        val description: String,
        @JsonIgnore
        @ManyToOne
        val creator: User,
        @OneToMany(fetch = EAGER)
        @Fetch(SUBSELECT)
        val videos: MutableList<Video> = mutableListOf(),
        @Id
        @GeneratedValue(strategy = SEQUENCE)
        val id: Long = 0
)

My code for persisting looks as the following

@Singleton
@Transactional
class ExerciseServiceDefault(override val repository: ExerciseRepository,
                             private val entityManager: EntityManager) : ExerciseService {

    override fun update(id: Long, exercise: Exercise): Exercise {
        val existing = get(id)
        val new = Exercise(exercise.name, exercise.description, existing.creator, existing.videos, existing.pictures, id)
        return repository.save(new)
//        return entityManager.merge(new)
    }
...

If I change the above code to use entityManager.merge(new) everything works just fine. But I'd rather not have to inject the entityManager.

Any clue about how I can make the updating happen using my repository?

I've tried adding cascade = [MERGE] to my relations to no avail.