组合就是强聚合 ，聚合就是双向的多对一，一对多

1.一对多配置

• cascade = CascadeType.ALL:包含所有级联(增删改)
• orphanRemoval = true:孤儿删除
• mappedBy = "外键字段名"：放弃关系维护，提高性能

@OneToMany(mappedBy = "purchasebill",cascade = CascadeType.ALL,orphanRemoval = true)
private List<Purchasebillitem> purchasebillitems = new ArrayList<>();

2.多对一配置

• fetch = FetchType.LAZY：懒加载，使用的时候才去查询，提高性能
• JsonIgnore：避免Json循环

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "bill_id")
@JsonIgnore
private Purchasebill purchasebill;