虚拟视图

多表关联常常通过创建数据库视图的方式来解决，其实也可以使用虚拟视图，维护更为方便。

表person和表id_card通过外键id_card_id关联。

创建虚拟视图PersonIdCard类：

@Entity
@Immutable
@Subselect("SELECT p.id id, p.name name, p.age age, ic.number number " +
           "FROM person p " +
           "LEFT JOIN id_card ic " +
           "ON p.id_card_id=ic.id")
public class PersonIdCard {
    @Id
    private Long id;
    private String name;
    private Integer age;
    private String number;

    // getter and setter}

public interface PersonIdCardRepository extends JpaRepository<PersonIdCard, Long>, JpaSpecificationExecutor<PersonIdCard> {
}

public List<PersonIdCard> findAll(SearchRequest request) {
    Specification<PersonIdCard> specification = new Specifications<PersonIdCard>()
            .gt(Object.nonNull(request.getAge()), "age", 20)
            .build();

    return personRepository.findAll(specification); 
}