How to map ONE-TO-MANY native query result into a POJO class using #SqlResultSetMapping - one-to-many

Question

Im working in a backend API using Java and MySql, and I'm trying to use #SqlResultSetMapping in JPA 2.1 for mapping a ONE-TO-MANY native query result into a POJO class, this is the native query:
#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")
And, here is my result SQL mapping:
#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")
}
)
})
And, here is the implementation of the POJO classes: (I just removed the constructor and the getters/setter)
MiniUser class
public class MiniUser {
String id;
String religionId;
Gender gender;
List<MiniUserAnswer> answers;
String name;
String city_id;
}
and the MiniUserAnswer class
public class MiniUserAnswer {
String questionId;
String answerId;
}
My goal is to execute this Query and return a list of MiniUser, and in each MiniUser: a list of his “answers", which is a list of MiniUserAnswer.
after running this code, I got this error:
The column result [answers] was not found in the results of the query.
I know why, it's because there is no “answers" field in the query select statement.
So, how can I accomplish something like this, considering the performance? This answers list may reach 100.
I really appreciate your help, Thanks in advance!

Answer 1

The query "SELECT DISTINCT t1.ID, t1.RELIGION_ID t1.gender, t1.NAME, t1.CITY_ID, t2.question_id, t2.answer_id" does not return a parameter called answers.
To obtain the result you are looking for I would use:
Option 1 (Criteria Builder)
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();
Option 2 (Named query, not native)
#NamedQuery(name = "User.getAll”, query = "SELECT t1 FROM UserTableEntityt1 join fetch t1.answers)
Option 3 (Entity subgraph, new in JPA 2.1)
In User Entity class:
#NamedEntityGraphs({
#NamedEntityGraph(name = "graph.User.Answers", attributeNodes = #NamedAttributeNode("answers"))
})
In DAO set hints in the entity manager:
EntityGraph graph = this.em.getEntityGraph("graph.User.Answers");
Map hints = new HashMap();
hints.put("javax.persistence.fetchgraph", graph);