Java 在 JOOQ 中获取按外部字段分组的 POJO

以多对多(book_author)关系中的book和author个表为例.

我目前有这个，而且很管用:

Map<UUID, List<Book>> booksByAuthorId = dsl
    .select(BOOK_AUTHOR.AUTHOR_ID, BOOK_AUTHOR.book())
    .from(BOOK_AUTHOR)
    .where(BOOK_AUTHOR.AUTHOR_ID.in(authorIds))
    .fetchGroups(
        BOOK_AUTHOR.AUTHOR_ID,
        mapping((a, bookRecord) -> mapping(Book::new).apply(bookRecord)));
   

但嵌套的mapping让我觉得我做错了什么.这真的是最明显的方法吗？

如果我用multiset重写它，基于this example ，我希望它看起来是这样的:

select(
        AUTHOR.ID,
        multiset(
            select(BOOK_AUTHOR.book())
            .from(BOOK_AUTHOR)                                        
            .where(BOOK_AUTHOR.AUTHOR_ID.eq(AUTHOR.ID)))
            .as("books").convertFrom(r -> r.map(mapping(Book::new)))
        .from(AUTHOR)
        .fetchMap(...)

但是，与我的第一个示例类似，类型根本不对齐，因为映射器接收Record1<BookRecord>，而预期接收BookRecord.因此，我不得不再次编写嵌套映射:

dsl.select(
        AUTHOR.ID,
        multiset(
            select(BOOK_AUTHOR.book())
            .from(BOOK_AUTHOR)
            .where(BOOK_AUTHOR.AUTHOR_ID.eq(AUTHOR.ID)))
            .as("books")
            //? seems wrong
            .convertFrom(r -> r.map(mapping((b) -> mapping(Book::new).apply(b)))))
        .from(AUTHOR)
        .fetchMap(AUTHOR.ID, mapping((a, b) -> b));

编辑:

以下是我基于卢卡斯的答案对MultiSet的最后看法:

dsl.select(
    AUTHOR.ID,
    multiset(
            select(BOOK_AUTHOR.book().mapping(Book::new))
            .from(BOOK_AUTHOR)
            .where(BOOK_AUTHOR.AUTHOR_ID.eq(AUTHOR.ID)))
            .as("books").convertFrom(r -> r.map(Record1::value1)))
    .from(AUTHOR)
    .fetchMap(AUTHOR.ID, Record2::value2);