MySql - 一对多连接，将 JOIN 与 JDBI 映射到列表答案

【问题标题】：MySql - One to Many Join, mapping JOIN with JDBI to ListMySql - 一对多连接，将 JOIN 与 JDBI 映射到列表
【发布时间】：2019-01-15 16:11:19
【问题描述】：

我正在尝试使用 JDBI 编写一个对象查询，该查询将从左表中获取完整的一行，并将右表中的所有匹配行作为一个列表连接起来（一个作者可以有很多本书）。

作者 id, name, createdAt, updatedAt, email, phoneNumber

书 id, title, authorId, categories, createdAt, updatedAt

我要创建的最终对象的结构如下：

class AuthorWithBooks() {
  int id,
  String name,
  List<String> categories,
  long createdAt,
  long updatedAt,
  String email,
  String phoneNumber
  List<Book> books
}

书在哪里：

class Book {
  id,
  title,
  authorId,
  categories,
  createdAt,
  updatedAt
}

这是我正在尝试的查询（不按原样抓取书籍）

@SqlQuery("SELECT " + AUTHOR_COLUMN MAMES + ", " + BOOK_COLUMN_NAMES + " FROM authors as author" +
      " LEFT JOIN books AS book" +
      " ON author.id = book.authorId" +
      " WHERE id = :authorId")
  List<AuthorWithBooks> getAuthorWithBooks(@Bind("authorId") int authorId);

将不胜感激任何帮助/有人指出我正确的方向！

谢谢！

【问题讨论】：

标签： java mysql jdbi

【解决方案1】：

看来你需要@UseRowReducer

您的示例的实现如下所示：

@SqlQuery("SELECT a." + AUTHOR_COLUMN MAMES + ", b." + BOOK_COLUMN_NAMES + " FROM authors as author" +
  " LEFT JOIN books AS book" +
  " ON author.id = book.authorId" +
  " WHERE id = :authorId")
@RegisterBeanMapper(value = Book.class, prefix = "b") 
@RegisterBeanMapper(value = AuthorWithBooks.class, prefix = "a")
@UseRowReducer(AuthorBookReducer.class) 
List<AuthorWithBooks> getAuthorWithBooks(@Bind("authorId") int authorId);

class AuthorBookReducer implements LinkedHashMapRowReducer<Integer, Author> { 
    @Override
    public void accumulate(Map<Integer, Author> map, RowView rowView) {
        Author author = map.computeIfAbsent(rowView.getColumn("a_id", Integer.class), 
                                       id -> rowView.getRow(Author.class));

        if (rowView.getColumn("b_id", Integer.class) != null) { 
            author.getBooks().add(rowView.getRow(Book.class));
        }
    }
}

【讨论】：