表单与数据库表之间的组合关系

//一方的配置
/**
    cascade = CascadeType.ALL:包含所有级联(增删改)
    orphanRemoval = true:孤儿删除
    mappedBy = "bill"：放弃关系维护
*/
@OneToMany(cascade = CascadeType.ALL, mappedBy = "bill", fetch = FetchType.LAZY, orphanRemoval = true)
private List<Purchasebillitem> items = new ArrayList<Purchasebillitem>();

//多方的配置  optional = false ：非空
@ManyToOne(fetch = FetchType.LAZY, optional = false)
@JoinColumn(name = "bill_id")
@JsonIgnore
private Purchasebill bill;