在spring数据中保存父项时如何引用子项id

@Entity
public class ParentItem {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // 其他属性...

    @ManyToOne
    @JoinColumn(name = "child_item_id")
    private ChildItem childItem;

    // getter和setter方法...
}

@Entity
public class ChildItem {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // 其他属性...

    @OneToMany(mappedBy = "childItem")
    private List<ParentItem> parentItems;

    // getter和setter方法...
}

ParentItem parentItem = new ParentItem();
// 设置其他属性...

ChildItem childItem = childItemRepository.findById(childItemId).orElse(null);
parentItem.setChildItem(childItem);

parentItemRepository.save(parentItem);