假設(shè)有兩個表，訂單表和產(chǎn)品表，訂單跟產(chǎn)品的關(guān)系是一對多的關(guān)系，那么在JPA中怎樣表示一對多的關(guān)系呢？實(shí)體關(guān)系一對多映射有兩種方式：

1 外鍵關(guān)聯(lián)

//訂單表
@Entity
@Table(name = "orders")
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @OneToMany(cascade = {CascadeType.ALL})
    @JoinColumn(name = "order_id")
    private List<Product> productList;

    ...
}

//產(chǎn)品表
@Entity
@Table(name = "product")
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    ...

這樣在表product中會增加一列order_id

2 表關(guān)聯(lián)

//訂單表
@Entity
@Table(name = "orders")
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @OneToMany(cascade = {CascadeType.ALL})
    @JoinTable(name = "order_has_product", joinColumns = {@JoinColumn(name = "order_id", referencedColumnName = "id")},inverseJoinColumns = {@JoinColumn(name = "product_id", referencedColumnName = "id")})
    private List<Product> productList;

    ...
}

這樣在product表中不會增加任何外鍵，而是新建了一張order_has_product表