JPA，实体管理器，选择许多列并获取结果列表自定义对象 [英] JPA, Entity manager, select many columns and get result list custom objects

问题描述

  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：

$ p $ 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（）;


我需要一张带有类别id和产品数量的地图。

$ b $不幸的是，JPA没有提供一个标准方法来检索 Map 中的结果。但是，通过遍历结果列表来手动构建地图很简单：

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

How I can get list of custom objects, like results below query:

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

By example:

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();

I need a map with category id and number of products in category.

解决方案

Unfortunately, JPA doesn't provide a standard way to retrieve the results in a Map. However, building up your map manually by walking through the result list is simple enough:

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]);

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