JPA 继承例子 – IT问题

问题：

Entity Car 有两类Entity BigCar 及Entity SmallCar；数据库中对应三张表 car 、small_car、big_car 。

采用 @OneToOne 之类的做法会造成 query list 时 N 次查询问题（如 car list 有 10条，则需要查询 11 次 sql)

解决：

可以调整 Entity 通过 JPA 继承的方式解决

@Data
@Table(name = "car")
@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "type", discriminatorType = DiscriminatorType.STRING)
public class Event implements Serializable {

    @Serial
    private static final long serialVersionUID = 1L;

    @Id
    @Column(name = "id", nullable = false)
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer eventId;
}



@Data
@Entity
@Table(name = "small_car")
@DiscriminatorValue("smallcar")
@PrimaryKeyJoinColumn(name="car_id")
public class SmallCar extends Car {
...
}


@Data
@Entity
@Table(name = "big_car")
@DiscriminatorValue("bigcar")
@PrimaryKeyJoinColumn(name="car_id")
public class BigCar extends Car {
...
}

@Data

@Table(name = "car")

@Entity

@Inheritance(strategy = InheritanceType.JOINED)

@DiscriminatorColumn(name = "type", discriminatorType = DiscriminatorType.STRING)

public class Event implements Serializable {

@Serial

private static final long serialVersionUID = 1L;

@Id

@Column(name = "id", nullable = false)

@GeneratedValue(strategy = GenerationType.IDENTITY)

private Integer eventId;

}

@Data

@Entity

@Table(name = "small_car")

@DiscriminatorValue("smallcar")

@PrimaryKeyJoinColumn(name="car_id")

public class SmallCar extends Car {

...

}

@Data

@Entity

@Table(name = "big_car")

@DiscriminatorValue("bigcar")

@PrimaryKeyJoinColumn(name="car_id")

public class BigCar extends Car {

...

}

这样 query list 会使用 join 的方式一条sql 取出结果

参考：

《Pro JPA2 中文版: 精通Java 持久化API》 p289