使用 entityManager 加载集合

Question

我有啤酒

@Entity
@Table(name="beer")
public class Beer implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue
    private Long id;
    private String brewery;
    private String name;

    ...

}

一瓶

@Entity
@Table(name="bottle")
public class Bottle {

    @Id
    @GeneratedValue
    private Long id;
    private BottleSize size;

    @ManyToOne
    private Beer beer;

    ...
}

还有道

public class BottleDao {

    private EntityManager entityManager;

    public void setEntityManager(EntityManager entityManager) {
        this.entityManager = entityManager;
    }

    public void saveBottle(Bottle bottle) {
        entityManager.persist(bottle);
    }

    public List<Bottle> findByBeer(Beer beer) {
        Bottle foundBottle = entityManager.find(Bottle.class, beer);
        return null;
    }

    public Bottle findById(Long id) {
        Bottle foundBottle = entityManager.find(Bottle.class, id);
        return foundBottle;
    }

}

如果我跑了

@Test
public void testFindByBeer() throws Exception {
    Beer pils = createHoepfnerPilsner();
    beerDao.saveBeer(pils);
    Beer hefe = createHoepfnerHefe();
    beerDao.saveBeer(hefe);

    Bottle bottlePils = createBottle().withBeer(pils).create();
    bottleDao.saveBottle(bottlePils);
    bottleDao.saveBottle(createBottle().withBeer(hefe).create());

    flushAndClear();
    List<Bottle> bottles = bottleDao.findByBeer(pils);
    assertThat(bottles.size(), is(1));
    assertThat(bottles.get(0).getId(), is(bottlePils.getId()));
}

我收到错误 java.lang.IllegalArgumentException：为类 Bottle 提供了错误类型的 id。预期：类 java.lang.Long，得到类 Beer

我错过了什么？

Answer 1

您缺少的是 EntityManager.find() 返回由给定 ID（主键）标识的实体。并且一瓶啤酒不能识别。它由 Long 类型的 ID 标识。

你需要的是一个 JPQL 查询：

String jpql = "select bottle from Bottle bottle where bottle.beer = :beer";
List<Bottle> bottles = em.createQuery(jpql, Bottle.class)
                         .setParameter("beer", pils)
                         .getResultList();