jpa没有返回任何带有appengine的行

Question

我正在使用以下代码从db

获取数据

public List<Quiz> getQuizes(String category, long level) {
    LOGGER.info("category = " + category + " level = " + level);
    EntityManager em = getEntityManager();
    try{
    CriteriaBuilder cb = em.getCriteriaBuilder();
    CriteriaQuery cq = cb.createQuery();
    Root e = cq.from(Quiz.class);
    cq.where(cb.equal(e.get("category"), cb.parameter(String.class)), cb.equal(e.get("level"), cb.parameter(Integer.class)));
    Query query = em.createQuery(cq);
    query.setParameter(1, category);
    query.setParameter(2, level);
    List quizes = query.getResultList();
    LOGGER.info("number of results found = " +  quizes.size());
    return quizes;
    }finally{
        em.close();
    }
}


@Entity
public class Quiz {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long quizId;
    private String question;
    private String answer;
    private String hint;
    private String category;
    private int level;
    private String imageUrl;
    private String status ;
    private String optionA;
    private String optionB;
    private String optionC;
    private String optionD;
    private String wikiLink;

   //setters , and getters
}

即使数据库中存在过滤条件数据，我也总是得到0条记录。为什么？