@Entity
public class Parent
{
	@Id
	@GeneratedValue
	private int id;

	@OneToMany(mappedBy = "parent", orphanRemoval = true, cascade = CascadeType.ALL)
        @BatchSize(size = 20)
	private List<Child> children = new ArrayList<Child>();

       public void addChild(Child child) {
        child.setParent(this);
        this.children.add(child);
      }
}

@Entity
public class Child
{
	@Id
	@GeneratedValue
	private int id;

	@ManyToOne(optional = false, fetch = FetchType.LAZY)
        @JoinColumn(name = "parent_id", referencedColumnName = "id", nullable = false, updatable = false)
	private Parent parent;
}

        Parent existingParent = parentRepository.findById(PARENT_ID).get();
        Child newChild= new Child ();
        existingParent .addChild(newChild);
        //when
        Parent savedParent  = parentRepository.save(existingParent);

        Parent existingParent = parentRepository.findById(PARENT_ID).get();
        Child newChild= new Child ();
        existingParent.getChildren().size();
        existingParent .addChild(newChild);
        //when
        Parent savedParent  = parentRepository.save(existingParent);

protected void entityIsPersistent(MergeEvent event, Map copyCache) {
		LOG.trace( "Ignoring persistent instance" );

		//TODO: check that entry.getIdentifier().equals(requestedId)

		final Object entity = event.getEntity();
		final EventSource source = event.getSession();
		final EntityPersister persister = source.getEntityPersister( event.getEntityName(), entity );

		( (MergeContext) copyCache ).put( entity, entity, true );  //before cascade!

		cascadeOnMerge( source, persister, entity, copyCache );
                // here we have two elements in collection, one Child that has been persisted in database, and the same Child object, before it persisted in database.
		copyValues( persister, entity, entity, source, copyCache );
                // here we have two the same Child object that have been persisted in database
		event.setResult( entity );
	}