Question

我正在使用PLAY 2.2.1，Ebean构建WebApp，并希望通过ManyToMany关系优化获取与另一个实体关联的实体列表。

更确切地说，我正在获取一系列书籍：

@Entity
public class Book extends Model {

    @Id
    @Column(name="book_id")
    public int bookId;

    @ManyToMany
    @JoinTable(name="book_author", 
        joinColumns={@JoinColumn(name="book_id", referencedColumnName = "book_id")},
        inverseJoinColumns={@JoinColumn(name="author_id", referencedColumnName = "author_id")})
    public List<Author> authors;
}

...每本书都与作者列表相关联：

@Entity
public class Author extends Model {

    @Id
    @Column(name="author_id")
    public int authorId;
}

创建查询并获取其结果的代码是

Query<Book> queryBooks = Ebean.createQuery(Book.class);
queryBooks.where("...");
FutureList<Book> fLBooks = queryBooks.findFutureList();

if (fLBooks.isDone()){
    List<Book> books = fLBooks.get();
}

FutureList，因为我想并行说明不同的查询。我需要访问所有书籍和所有作者，因此我必须遍历所有书籍及其所有作者：

for(Book b : books){
    List<Author> authors = b.authors;
    for (Author a : authors){
        ...
    }
}

它有效，但内循环中的迭代非常慢，比分别查询表book，book_author和author要慢得多。我想Ebean一次取一个作者。有没有更快的方法呢？

使用@ManyToMany(fetch=FetchType.EAGER)不会改变检索速度。

更新05.02.2014

我以这种方式创建查询：

Query<Book> queryBooks = Ebean.createQuery(Book.class);
FutureList<Book> fLBooks = queryBooks.fetch("authors", "*").where("...").findFutureList();

生成的Ebean查询是

select 
t0.book_id c0, 
t1.author_id c0
from book t0 
left outer join book_author t1z_ on t1z_.book_id = t0.book_id  
left outer join author t1 on t1.author_id = t1z_.author_id  
where MATCH (t0.text) AGAINST ('...' IN BOOLEAN MODE) order by t0.book_id

好： Ebean现在生成单个查询。错误还比分别查询表book，book_author和author慢20倍。也许mysql查询优化器不起作用？

Answer 1

嗯，为什么不使用普通的Ebean Finder并且自己与@JoinColumn这样的事情打架？ Ebean的优势在于保持事物非常简单，以正确方式编写的相同模型是：

图书

@Entity public class Book extends Model { @Id public Integer id; public static Finder<Integer, Book> find = new Finder<>(Integer.class, Book.class); public String title; @ManyToMany public List<Author> authors; }

<强>作者

@Entity public class Author extends Model { @Id public Integer id; public static Finder<Integer, Author> find = new Finder<>(Integer.class, Author.class); public String name; @ManyToMany(mappedBy = "authors") // take care about this mappedBy annotation, otherwise Ebean will want to use second JoinColumn public List<Book> books; }

接下来将SQL日志记录添加到application.conf
中
db.default.logStatements = true logger.com.jolbox = DEBUG

并比较执行：

String out = ""; List<Book> books = Book.find.findList(); for (Book book : books) for (Author author : book.authors) out += "Book " + book.title + " by " + author.name + "\n";

与

String out = ""; List<Book> books = Book.find.fetch("authors", "*").findList(); for (Book book : books) for (Author author : book.authors) out += "Book " + book.title + " by " + author.name + "\n";

一般来说，你是对的，第一种方式是为每本书执行额外的查询，第二种方法只执行一次查询。

反方向：

String out = ""; List<Author> authors = Author.find.findList(); for (Author author : authors) for (Book book : author.books) out += author.name + " wrote this book: " + book.title + "\n";

与

String out = ""; List<Author> authors = Author.find.fetch("books", "*").findList(); for (Author author : authors) for (Book book : author.books) out += author.name + " wrote this book: " + book.title + "\n";

Ebean ManyToMany：如何优化ManyToMany关系的获取

1 个答案: