Question

I am using JPA in my web application and when I do not want the DB to return all of the object I do something like:

    Query query = super.entityManager.createQuery("select b.id, b.buyName from Buy b where b.id in (:list)");
    query.setParameter("list", buyIds);

    List<Object[]> result = query.getResultList();
    List<Buy> retval = new ArrayList<Buy>();

    for (Object[] item : result) {
        Buy buy = new Buy();
        buy.setId((Long)item[0]);
        buy.setBuyName((String)item[1]);
        retval.add(buy);
    }

    return retval;

That is , querying the fields I want and then manually creating a list of the desired object and populating it.
Is this a good practice? It feels like there is a better way of doing this.

Thank you.

Answer 1

It’s possible to directly return a new object from the query, with just the information that you need. For instance, your query would look like this:

select new BuyDTO(b.id, b.buyName) from Buy b where b.id in (:list) 

And the data transfer object would include only the needed fields:

public class BuyDTO {

    private Long id;
    private String buyName;

    public BuyVO(Long id, String name) {
        this.id = id;
        this.buyName = name;
    }

    // getters and setters

}

When the query is run, it will return a List<BuyDTO>