使用JPA选择元组的好方法 [英] Nice way to select a tuple using JPA

问题描述

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();

这将显示错误"Cannot create TypedQuery for query with more than one return".我可以通过省去类型参数(并使用Object []代替Tuple，稍后再发现)来解决此问题:

This gives the error "Cannot create TypedQuery for query with more than one return". I could work around that by leaving out the type parameter (and using Object[] instead of Tuple, as I later found out):

@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();

但是有没有不需要未经检查的代码的解决方案?

But is there a solution that doesn't require unchecked code?

推荐答案

元组实际上不是比数组更类型安全的，不是吗?

A tuple isn't really any more typesafe than an array, is it?

您可以在这里使用构造函数表达式.在我的头顶上，就像这样:

What you could do here is use a constructor expression. Off the top of my head, this is something like:

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，但这通常很简单.

This requires you to write a new class to hold the result, but this will typically be pretty trivial.

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