@Entity
public class Parent implements Serializable {

	@Id
	private Integer id;

	@OneToMany(fetch = FetchType.EAGER, mappedBy = "parent", orphanRemoval = true, cascade = CascadeType.ALL)
	private Set<Child> children = new HashSet<>();

        // getters, setters, hashCode, equals omitted for brevity
}

@Entity
public class Child implements Serializable {

	@Id
	@ManyToOne(fetch = FetchType.LAZY, optional = false)
	private Parent parent;

	@Id
	private String value;

        // getters, setters, hashCode, equals omitted for brevity
}

EntityManager entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();

Parent parent = new Parent();
parent.setId(10);
Child child = new Child();
child.setParent(parent);
child.setValue("old");
parent.getChildren().add(child);

entityManager.persist(parent);
entityManager.flush();
entityManager.detach(parent);
entityManager.getTransaction().commit();
entityManager.close();

// 'parent' is now detached; now let's change it and merge
Child oldChild = parent.getChildren().iterator().next();
Child newChild = new Child();
newChild.setParent(parent);
newChild.setValue("new");
parent.getChildren().add(newChild);
parent.getChildren().remove(oldChild);

entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();
entityManager.merge(parent);
entityManager.getTransaction().commit();
entityManager.close();

Caused by: org.h2.jdbc.JdbcSQLException: NULL not allowed for column "VALUE"; SQL statement:
insert into Child (value, parent_id) values (?, ?) [23502-176]