假設有兩個表，訂單表和產品表，訂單跟產品的關系是一對多的關系，那么在JPA中怎樣表示一對多的關系呢？實體關系一對多映射有兩種方式：
1 外鍵關聯
//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 表關聯
//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
