JPA-join使用Root，join获取字段

Question

我有这些类，字段总是延迟加载的。

public class Book {
     @ManyToOne(fetch = FetchType.LAZY)
     private Publication publication;
     @OneToMany(fetch = FetchType.LAZY)
     private List<Author> authors;
     @OneToMany(fetch = FetchType.LAZY)
     private List<Genre> genres;
}
public class Publication{
     @OneToMany(fetch = FetchType.LAZY)
     private List<Founder> founders;
}

我试图实现的是在连接获取上进行连接获取。在hql中，它看起来是这样的：

FROM Book b LEFT JOIN b.authors LEFT JOIN b.genres LEFT JOIN b.publication p LEFT JOIN p.founders

这是我迄今为止尝试过的，但没有奏效：

Root<Book> root ...
Join<Book, Publication> publication = root.join("publication");
publication.fetch("founders");
root.fetch("authors");
root.fetch("genres");

以下是异常的原因：

Caused by: javax.persistence.PersistenceException: org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1763)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1677)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1683)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.createQuery(AbstractEntityManagerImpl.java:588)
    at org.hibernate.jpa.criteria.CriteriaQueryImpl$1.buildCompiledQuery(CriteriaQueryImpl.java:336)
    at org.hibernate.jpa.criteria.compile.CriteriaCompiler.compile(CriteriaCompiler.java:147)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.createQuery(AbstractEntityManagerImpl.java:736)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)

Answer 1

JPA的规范说：

一个本规范的实施。使用的应用程序多级fetch联接将不可移植。

尝试减少从查询中提取联接，稍后（在数据选择之后）初始化数据。您可以在同一事务中使用size（）或Hibernate.initialize（）初始化延迟加载的数据，例如：

publication.getFounders().size();

或

Hibernate.initialize(publication.getFounders());