我正在尝试使用动态排序，但检索到的列表未排序 [英] I am trying to use dynamic order by but the list retrieved is not ordered

问题描述

public List<Series> findSeries(int period, String fieldname, int num) {
    TypedQuery<Series> query = em.createQuery(
            "select s from Series s where  s.period = ?1 order by ?2",
            Series.class);
    query.setParameter(1, period);
    query.setParameter(2, fieldname);
    query.setMaxResults(num);
    return query.getResultList();
}

这是我正在使用的方法.我认为 order by 甚至没有被执行，即使我传递了不正确的字段名，它也不会给出任何错误.

This is the method I am using. I think order by isn't even getting executed, it doesn't give any error even when I pass incorrect fieldname.

推荐答案

当涉及到动态限制和排序时，最好使用 PagingAndSortingRepository，所以现在我的 Repository 扩展了这个存储库.我可以简单地使用 JPA 条件查询，如下所示.

When it comes to dynamic limit and ordering, its best to use PagingAndSortingRepository so now my Repository extends this repository. I can simply use JPA criteria query as below.

如果您想了解有关 JPA 标准查询的更多信息，我发现这非常有用 http://docs.spring.io/spring-data/data-jpa/docs/1.0.x/reference/html/#jpa.query-methods.query-creation

If u want to learn more about JPA criteria query i found this very helpful http://docs.spring.io/spring-data/data-jpa/docs/1.0.x/reference/html/#jpa.query-methods.query-creation

 @Repository
public interface SeriesRepository extends PagingAndSortingRepository<Series,Long>{
List<Series> findByPeriod(int period, Pageable pageable);
}

然后，当我从我的 dao 调用此方法时，我可以实例化 PageRequest，它是 Pageable 的实现之一.我可以为这个实例添加限制和排序顺序.

And then when I call this method from my dao i can just instantiate PageRequest which is one of the implementation of Pageable. I can add limit and sorting order to this instance.

public List<Series> getSeriesByFilter(int period, String fieldname, int num) {
    Sort sort = new Sort(Sort.Direction.ASC, fieldname);
    Pageable pageable = new PageRequest(0, num, sort);
    return seriesRepo.findByPeriod(period, pageable);
}

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