how to get list of long values in hibernate from the table while casting from bigint to long

问题

I get the following exception when using the getEvents method:

org.hibernate.MappingException: Unknown entity: java.lang.Long

public List<Long> getEvents(Person person) {

    String q = "select new java.lang.Long(te.event_id) "
        + "from teachers_event te"
        + "where te.teachers_id = :personId ";
    Query query = entityManager.createNativeQuery(q, Long.class);
    query.setParameter("personId", person.getId());
    return (List<Long>) query.getResultList();
}

The teachers_event table in database connects the id of teacher with the event. I don't map this table to an entity. I only want to get the ids from it. The ids in the PostgreSQL table are bigint. Is there any way to achieve it without making a separate class to map the TeacherEvents entity?

回答1:

In JPA 2.1 there is a feature called "result set mapping".

Basically you have to define a POJO class which would hold the result values (all the values must be passed using the constructor):

public class IdResult{

    private Object id;

    public IdResult(Object id){
        this.id = id;
    }

     public Long getLongId(){
        return (Long)id;
     }
}

Then you have to declare the mapping on one of your entities (does not matter on which, it just has to be a declated @Entity):

@SqlResultSetMapping(name="IdMapping", classes = {
    @ConstructorResult(targetClass = IdResult.class, 
    columns = {@ColumnResult(name="id")})
})

You would have to use an alias in your query to match the @ColumnResult name:

select te.event_id as id
...

And finally use in the query creation:

Query query = entityManager.createNativeQuery(q, IdMapping.class);
return (List<IdResult>) query.getResultList();

Then in your service layer you would just use getLongId();

来源：https://stackoverflow.com/questions/47629670/how-to-get-list-of-long-values-in-hibernate-from-the-table-while-casting-from-bi

标签

java

sql

hibernate

hql

hibernate-native-query