Java 如何在Hibernate/JPA中使用@Formula

我试图通过下面一段简单的代码来了解

@Formula

注释是如何工作的

我能够打印描述和bidAmount列的值，但用@Formula注释的字段，即shortDescription和averageBidAmount返回空值

有人能帮我指出这里的代码有什么问题吗

我正在Mac OSX上使用Hibernate 5.0、postgresql-9.3-1102-jdbc41和TestNG

    import java.math.BigDecimal;
    import java.util.List;
    import javax.persistence.Entity;
    import javax.persistence.EntityManager;
    import javax.persistence.EntityManagerFactory;
    import javax.persistence.GeneratedValue;
    import javax.persistence.Id;
    import javax.persistence.Persistence;
    import javax.transaction.UserTransaction;
    import org.testng.annotations.Test;
    import com.my.hibernate.env.TransactionManagerTest;

    public class DerivedPropertyDemo extends TransactionManagerTest {

      @Test
      public void storeLoadMessage() throws Exception {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("HelloWorldPU");
        try {
          {
            UserTransaction tx = TM.getUserTransaction();
            tx.begin();
            EntityManager em = emf.createEntityManager();

            DerivedProperty derivedProperty1 = new DerivedProperty();
            derivedProperty1.description = "Description is freaking good!!!";
            derivedProperty1.bidAmount = BigDecimal.valueOf(100D);

            DerivedProperty derivedProperty2 = new DerivedProperty();
            derivedProperty2.description = "Description is freaking bad!!!";
            derivedProperty2.bidAmount = BigDecimal.valueOf(200D);

            DerivedProperty derivedProperty3 = new DerivedProperty();
            derivedProperty3.description = "Description is freaking neutral!!!";
            derivedProperty3.bidAmount = BigDecimal.valueOf(300D);

            em.persist(derivedProperty1);
            em.persist(derivedProperty2);
            em.persist(derivedProperty3);

            tx.commit();

            for (DerivedProperty dp : getDerivedProperty(em)) {
              System.out.println("============================");
              System.out.println(dp.description);
              System.out.println(dp.bidAmount);
              System.out.println(dp.getShortDescription());
              System.out.println(dp.getAverageBidAmount());
              System.out.println("#############################");
            }
            em.close();
          }
        } finally {
          TM.rollback();
          emf.close();
        }
      }


     public List<DerivedProperty> getDerivedProperty(EntityManager em) {
      List<DerivedProperty> resultList = em.createQuery("from " + DerivedProperty.class.getSimpleName()).getResultList();
      return resultList;
  }
}

编辑

我正在阅读《Hibernate第二版Java持久化》一书。

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谢谢

您的

DerivedProperty

实例是从持久性上下文返回的（从查询返回的结果集中只使用它们的ID）。这就是公式没有被评估的原因

---

如果不关闭实体管理器，则不会清除持久性上下文。提交第一个事务后，尝试添加

em.clear（）

，以强制清除持久性上下文。

Hack！！我不知道你们从哪里学到这些。我已经找了好几个小时了(

@Entity
class DerivedProperty {

  @Id
  @GeneratedValue
  protected Long id;

  protected String description;

  protected BigDecimal bidAmount;

  @org.hibernate.annotations.Formula("substr(description, 1, 12)")
  protected String shortDescription;

  @org.hibernate.annotations.Formula("(select avg(b.bidAmount) from DerivedProperty b where b.bidAmount = 200)")
  protected BigDecimal averageBidAmount;

  public String getShortDescription() {
    return shortDescription;
  }

  public BigDecimal getAverageBidAmount() {
    return averageBidAmount;
  }
}