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[英]For a JPA entity with many to 1 relation to a data entity, how to sort a list of objects by the result of a method call of the data entity?
[英]JPA, Entity manager, select many columns and get result list custom objects
我如何獲得自定義對象列表,如查詢下面的結果:
SELECT p.category.id, count(p.id) FROM Product p left join p.category c WHERE p.seller.id=:id GROUP BY c.id
舉例:
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();
我需要一張類別ID和類別中的產品數量的地圖。
不幸的是,JPA沒有提供在Map
檢索結果的標准方法。 但是,通過遍歷結果列表手動構建地圖非常簡單:
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]);
假設你正在使用hibernate(標記),可以嘗試下面的HQL查詢,我還沒有測試過。
SELECT new map(p.category.id as category_id, count(p.id) as id_count) FROM Product p left join p.category c WHERE p.seller.id=:id GROUP BY c.id
使用JPA 2.0和EclipseLink impl
對於第一個問題:自定義對象列表(沒有表對象):
回答:創建自定義模型並使用@Entity和@Id
@Entity
public class QueryModelDTO implements Serializable{
@Id
private Integer categoryId;
private int count;
---gets and sets
}
創建查詢並執行
QueryModelDTO qm = (QueryModelDTO) em.createQuery(
"SELECT p.category.id as categoryId, count(p.id) as count FROM Product p
left join p.category c WHERE p.seller.id=:id
GROUP BY c.id",QueryModelDTO.class)
.setParameter("id", id).getSingleResult();
第二個:如何在地圖上閱讀響應
回答:使用QueryHints和ResultTypes(這是@DannyMo答案的一個變體)
Query q = em.createNativeQuery("SELECT * FROM Foo f");
q.setHint(QueryHints.RESULT_TYPE, ResultType.Map);
List<Map> lm = q.getResultList();
for (Map map : lm) {
for (Object entry : map.entrySet()) {
Map.Entry<DatabaseField, Object> e = (Map.Entry<DatabaseField, Object>) entry;
DatabaseField key = e.getKey();
Object value = e.getValue();
log.debug(key+"="+value);
}
}
我希望這有幫助
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