JPA,实体管理器,选择许多列并获取结果列表自定义对象

Question

我如何获得自定义对象列表,如查询下面的结果:

SELECT p.category.id, count(p.id) FROM Product p left join p.category c WHERE p.seller.id=:id GROUP BY c.id

举例:

return getEntityManager().createQuery("SELECT p.category.id, count(p.id) FROM Product p left join p.category c WHERE p.seller.id=:id GROUP BY c.id").setParameter("id", id).getResultList();

我需要一张类别ID和类别中的产品数量的地图.

Answer 1

不幸的是,JPA没有提供检索结果的标准方法Map.但是,通过遍历结果列表手动构建地图非常简单:

TypedQuery<Object[]> q = getEntityManager().createQuery(
    "SELECT c.id, count(p.id) " +
    "FROM Product p LEFT JOIN p.category c " +
    "WHERE p.seller.id = :id " +
    "GROUP BY c.id", Object[].class).setParameter("id", id);

List<Object[]> resultList = q.getResultList();
Map<String, Long> resultMap = new HashMap<String, Long>(resultList.size());
for (Object[] result : resultList)
  resultMap.put((String)result[0], (Long)result[1]);