@Entity(name = "parent")
@Table(name = "parent")
public class Parent ... {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "parent_id_seq")
    @SequenceGenerator(name = "parent_id_seq", sequenceName = "parent_id_seq", allocationSize = 100)
    @Column(name = "id", nullable = false)
    private Long id;

    @JoinColumn(name = "child", referencedColumnName = "id", nullable = true)
    @ManyToOne(optional = true, cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private Child child;

    ....
}


@Entity(name = "child")
@Table(name = "child")
public class Child .... {
    
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "child_id_seq")
    @SequenceGenerator(name = "child_id_seq", sequenceName = "child_id_seq", allocationSize = 100)
    @Column(name = "id", nullable = false)
    private Long id;

    ...

}

entityManager.find(Child.class, id);

entityManager.clear();
entityManager.find(Child.class, id);

Parent p = entityManager.find(Parent.class, 1);

// The returned p has a Child with the id of 20

// later in the TX, in a totally different class where code scope only has access to the ID of the child, and knows nothing about the parent instance...

Child c = entityManager.find(Child.class, 20);
// c = a lazy reference, just like I'd get from entityManager.getReference(Child.class, 20);