java - 使用 JPA 和 Hibernate (NotNull) 映射集合的最新子项

Question

我需要从集合中获取最后一个实体元素。我正在使用@JoinFormula:

@Entity
public class Book {

    @ManyToOne
    @JoinFormula("(select * from
                     (SELECT r.id FROM review r WHERE r.book_id = id ORDER BY r.postedAt DESC)
                   where rownum = 1)")
    private Review 
    ...
}

它的效果非常棒，但前提是书有一些评论。否则找不到书。因为 Hibernate 将其转换为交叉连接并在 WHERE 语句中使用条件:

review_entity.id = 
 (select * from (SELECT r.id FROM review r WHERE r.book_id = id ORDER BY r.postedAt DESC) where rownum = 1)

这里是否有任何选项可以将 JoinFormula 转换为左连接或类似的东西？

select
        book0_.id as id1_0_0_,
        book0_.title as title2_0_0_,
        book0_.version as version3_0_0_,
        (SELECT
            r.id 
        FROM
            review r 
        where
            r.book_id = book0_.id 
        ORDER BY
            r.postedAt DESC LIMIT 1) as formula1_0_,
        review1_.id as id1_1_1_,
        review1_.book_id as book_id4_1_1_,
        review1_.comment as comment2_1_1_,
        review1_.postedAt as postedAt3_1_1_ 
    from
        Book book0_ 
    left outer join
        Review review1_ 
            on (
                SELECT
                    r.id 
            FROM
                review r 
            where
                r.book_id = book0_.id 
            ORDER BY
                r.postedAt DESC LIMIT 1
        )=review1_.id 
    where
        book0_.id=? 

Answer 1

我不确定你要做什么是个好主意。您的领域模型与一本书有很多评论的现实不符。我知道您只想访问最新的评论，但这可能最好作为单独的查询来完成。否则，您可以更新域模型以反射(reflect)现实，但仍然通过 Hibernate 的超惰性属性以高性能方式获取最后的评论。

"Extra-lazy" collection fetching - individual elements of the collection are accessed from the database as needed. Hibernate tries not to fetch the whole collection into memory unless absolutely needed (suitable for very large collections)

@Entity
public class Book {

    @OneToMany
    @LazyCollection(LazyCollectionOption.EXTRA)
    @OrderBy("...")
    private List<Review> reviews; //needn't be exposed via public API

    public Review getLatestReview(){
      return reviews.get(reviews.size() - 1); //or first if ordered desc
    }
}

Hibernate 的 @Where 子句也可以用作将集合限制为仅一个元素的替代方法。