Question

I have two models, Purchase and Tag.

A Purchase can have many Tags (One to Many relationship).

What is the best (Efficent/Cleanest/etc) way to return all purchases that have one or more tags? (The actual tag entities dont have to be returned).

I’m currently using a column in the purchase table to determine if it has been tagged but would like to remove it (See PurchaseDAO for it been used)

PurchaseController:

@RequestMapping(value = "purchases/tagged", method = RequestMethod.GET)
@ResponseBody
public final List<Purchase> getTagged()
{
    return RestPreconditions.checkNotNull(purchaseService.getTagged());
}

@RequestMapping(value = "purchases/pending", method = RequestMethod.GET)
@ResponseBody
public final List<Purchase> getPending()
{
    return RestPreconditions.checkNotNull(purchaseService.getPending());
}

PurchaseService:

@Service
public class PurchaseService implements IPurchaseService
{
    @Autowired
    private IPurchaseDAO purchaseDAO;

    public PurchaseService()
    {

    }

    @Transactional
    public List<Purchase> getAll()
    {
        return purchaseDAO.findAll();
    }

    @Transactional
    public List<Purchase> getPending()
    {
        return purchaseDAO.getPending();
    }

    @Transactional
    public List<Purchase> getTagged()
    {
        return purchaseDAO.getTagged();
    }
}

PurchaseDAO:

@Repository
public class PurchaseDAO extends AbstractJpaDAO<Purchase> implements IPurchaseDAO {

    @PersistenceContext
    EntityManager entityManager;

    public PurchaseDAO() {
        setClazz(Purchase.class);
    }

    public  List<Purchase> getPending() {
        return entityManager.createQuery("from Purchase where tagged = 0")
                .getResultList();
    }

    public  List<Purchase> getTagged() {
        return entityManager.createQuery("from Purchase where tagged = 1")
                .getResultList();
    }
}

Both my Purchase and Tag DAOs extend the following AbstractJpaDAO:

public abstract class AbstractJpaDAO<T extends Serializable> implements
        IAbstractJpaDAO<T> {

    private Class<T> clazz;

    @PersistenceContext
    EntityManager entityManager;

    public void setClazz(final Class<T> clazzToSet) {
        this.clazz = clazzToSet;
    }

    public T findOne(final Long id) {
        return entityManager.find(clazz, id);
    }

    public List<T> findAll() {
        return entityManager.createQuery("from " + clazz.getName())
                .getResultList();
    }

    public void save(final T entity) {
        entityManager.persist(entity);
    }
}

Thanks

JB Suggestion Update

After changing PurchaseDAO getTagged() to:

public  List<Purchase> getTagged() {
        return entityManager.createQuery("SELECT p FROM Puchase p INNER JOIN p.tags")
                .getResultList();
    }

I get following error:

TRACE [http-bio-8080-exec-3] o.s.w.c.s.AnnotationConfigWebApplicationContext [AbstractApplicationContext.java:322] Publishing event in Root WebApplicationContext: ServletRequestHandledEvent: url=[/PurchaseAPIServer/api/purchases/tagged]; client=[192.168.1.17]; method=[GET]; servlet=[PurchaseAPIServer]; session=[null]; user=[null]; time=[411ms]; status=[failed: org.springframework.dao.InvalidDataAccessApiUsageException: org.hibernate.hql.ast.QuerySyntaxException: Puchase is not mapped [SELECT p FROM Puchase p JOIN p.tags tag]; nested exception is java.lang.IllegalArgumentException: org.hibernate.hql.ast.QuerySyntaxException: Puchase is not mapped [SELECT p FROM Puchase p JOIN p.tags tag]]

Answer 1

select p from Purchase p inner join p.tags

The inner join just makes there is at least one tag for the purchase.