如何使用@SqlResultSetMapping将一对多的本地查询结果映射到POJO类中？

Question

我正在使用Java和MySql构建后端API，尝试在JPA 2.1中使用@SqlResultSetMapping将一对多的本地查询结果映射到POJO类。以下是本地查询：

@NamedNativeQuery(name = "User.getAll”, query = "SELECT DISTINCT t1.ID, t1.RELIGION_ID t1.gender,t1.NAME,t1.CITY_ID , t2.question_id, t2.answer_id  FROM user_table t1 inner join user_answer_table t2 on t1.ID = t2.User_ID“,resultSetMapping="userMapping")

这是我的SQL映射结果：

@SqlResultSetMapping(
            name = "userMapping",
            classes = {
                            @ConstructorResult(
                                    targetClass = MiniUser.class,
                                    columns = {
                                            @ColumnResult(name = "id"),
                                            @ColumnResult(name = "religion_id"),
                                            @ColumnResult(name = "gender"),
                                            @ColumnResult(name = "answers"),
                                            @ColumnResult(name = "name"),
                                            @ColumnResult(name = "city_id")

                                    }
                            ),

                            @ConstructorResult(
                                    targetClass = MiniUserAnswer.class,
                                    columns = {
                                            @ColumnResult(name = "question_id"),
                                            @ColumnResult(name = "answer_id")

                                    }
                              )

            })

这里是POJO类的实现：（我只删除了构造函数和getter/setter）

MiniUser类

public class MiniUser {

    String id;
    String religionId;
    Gender gender;
    List<MiniUserAnswer> answers;
    String name;
    String city_id;
}

还有MiniUserAnswer类

public class MiniUserAnswer {

    String questionId;
    String answerId;
}

我的目标是执行此查询并返回MiniUser列表，在每个MiniUser中：他的“answers”列表，这是一个MiniUserAnswer的列表。

运行此代码后，我得到了以下错误：

The column result [answers] was not found in the results of the query.

我知道原因在于查询选择语句中没有“answers”字段。
那么，我怎样才能实现这样的结果，并考虑性能？答案列表可能会有100个之多。
非常感谢您的帮助，提前致谢！

- Tamer Saleh

---

请问有人能回答吗？我在想的做法可行吗？ - Tamer Saleh

Answer 1

查询语句 "SELECT DISTINCT t1.ID, t1.RELIGION_ID t1.gender, t1.NAME, t1.CITY_ID, t2.question_id, t2.answer_id" 中没有返回名为 answers 的参数。 要获得您想要的结果，我建议使用以下方法： 选项1（条件构建器）

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<UserTableEntity> cq = cb.createQuery(UserTableEntity.class);
Root<UserTableEntity> rootUserTable = cq.from(UserTableEntity.class);
Join<UserTableEntity,UserAnswerTableEntity> joinAnswerTable = rootUserTable.join(rootUserTable_.id) // if the relationship is defined as lazy, use "fetch" instead of "join"
//cq.where() NO WHERE CLAUSE
cq.select(rootUserTable)

entityManager.createQuery(cq).getResultList();

选项2（命名查询，非本地）

@NamedQuery(name = "User.getAll", query = "SELECT t1 FROM UserTableEntityt1 join fetch t1.answers)

选项3（实体子图，在JPA 2.1中新增）

在用户实体类中：

@NamedEntityGraphs({
    @NamedEntityGraph(name = "graph.User.Answers", attributeNodes = @NamedAttributeNode("answers"))
})

在DAO中，在实体管理器中设置提示：

EntityGraph graph = this.em.getEntityGraph("graph.User.Answers");

Map hints = new HashMap();
hints.put("javax.persistence.fetchgraph", graph);

- JLazar0