使用JPA选择元组的好方法

Question

final List<Tuple> data =
                    em.createQuery("SELECT p.id AS i, p.membership AS m FROM Player p WHERE p.id IN :ids", Tuple.class)
                    .setParameter("ids", ids)
                    .getResultList();

这给出了错误" Cannot create TypedQuery for query with more than one return".我可以通过省略type参数(并使用Object []而不是Tuple来解决这个问题,我后来发现):

@SuppressWarnings("unchecked")
final List<Object[]> data =
                    em.createQuery("SELECT p.id AS i, p.membership AS m FROM Player p WHERE p.id IN :ids")
                    .setParameter("ids", ids)
                    .getResultList();

但是,是否有一个不需要未经检查的代码的解决方案？

Answer 1

元组实际上并不比数组更安全,是吗？

你在这里可以做的是使用构造函数表达式.在我的头顶,这是这样的:

class PlayerMembership {
    public final int id;
    public final MembershipType membership;
    public PlayerMembership(int id, MembershipType membership) {
        this.id = id;
        this.membership = membership;
    }
}

List<PlayerMembership> data =
                    em.createQuery("SELECT NEW nl.bart.PlayerMembership(p.id, p.membership) FROM Player p WHERE p.id IN :ids", PlayerMembership.class)
                    .setParameter("ids", ids)
                    .getResultList();

这需要您编写一个新类来保存结果,但这通常是非常简单的.