How can I map the result set of query to a different entity other than table

Question

I am trying to fetch data from db and mapping it to different entity but I get

java.lang.IllegalArgumentException: java.lang.ArrayIndexOutOfBoundsException: 0

My Table looks like

@Entity
@Table(name = "Student")
@NamedNativeQueries({
    @NamedNativeQuery(name = "findAll", query = "Select a.student_id as id, a.student_code as code from Student a")
    })
public class Student {

    @Id
    private Long student_id;
    private String student_code;
    private String student_user_id;
    private String student_first_name;

    //Some other fields, getters and setters
}

My BO Looks like

@Entity
public class Generic{

    @Id
    private Long id;
    private String code;
    private String user_id;

    //getters and setters
}

My DAO call class something like this

Query query = entityManager.createNamedQuery("findAll", Generic.class);
query.getResultList();

I get exception on

entityManager.createNamedQuery("findAll", Generic.class);

This is the stack trace I have

    Caused by: java.lang.IllegalArgumentException: java.lang.ArrayIndexOutOfBoundsException: 0
at org.hibernate.internal.AbstractSharedSessionContract.buildQueryFromName(AbstractSharedSessionContract.java:774)
at org.hibernate.internal.AbstractSharedSessionContract.createNamedQuery(AbstractSharedSessionContract.java:869)
at org.hibernate.internal.AbstractSessionImpl.createNamedQuery(AbstractSessionImpl.java:23)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:301)
at com.sun.proxy.$Proxy157.createNamedQuery(Unknown Source)

Answer 1

I am trying to fetch data from db and mapping it to a different entity but I get ArrayIndexOutOfBoundsException

But in the following line, you are not doing that, you are trying to get the list of Student entity list. And what's your different Entity here?

entityManager.createNamedQuery("findAll", Student.class);

But as you provided another entity Generic , I assume you want your data to be loaded in that. There are different possible solutions to this problem. Lets figure out.

Using the SELECT NEW keywords

Update your Native query to this

@NamedNativeQueries({
    @NamedNativeQuery(name = "Student.findAll", query = "SELECT NEW Generic(a.student_id, a.student_code) FROM Student a")
})

You have to define a Constructor in the Generic class as well that qualifies this call

public void Generic(Long id, String code) {
     this.id = id;
     this.code = code;
}

And update the query execution in DAO as

List<Generic> results = em.createNamedQuery("Student.findAll" , Generic.class).getResultList();

Note: You may have to place the fully qualified path for the Generic class in the Query for this

Construct from List<Object[]>

Alternatively the straight forward and simplest solution would fetch the result list as a list of Object[] and populate to any new Object you want.

List<Object[]> list = em.createQuery("SELECT s.student_id, s.student_code FROM Student s")
                        .getResultList(); 

for (Object[] obj : list){
    Generic generic = new Generic();
    generic.setId(((BigDecimal)obj[0]).longValue());
    generic.setCode((String)obj[1]);
}