为什么spring data jpa不发布JOIN查询？ [英] Why spring data jpa not issuing JOIN query?

问题描述

我用一对多关系创建了两个实体 Book 和 Book_Category 。当我发行 BookCategoryRepository.findAll（）时，我期望hibernate使用'INNER JOIN'查询。但它只是发出查询来从Book_Category中获取数据。

我缺少什么？我应该怎么做才能让hibernate发出JOIN查询？

Book.java

 公共类书籍{
 
 @Id 
 @GeneratedValue（strategy = GenerationType.AUTO）
 private int id; 
私人字符串名称; 
 
 @ManyToOne 
 @JoinColumn（name =book_category_id）
私人BookCategory bookCategory; 
} 
  

BookCategory.java

< pre $ @Entity
@Table（name =book_category）
public class BookCategory {

@Id
@GeneratedValue （strategy = GenerationType.AUTO）
private int id;
私人字符串名称;
@OneToMany（mappedBy =bookCategory，fetch = FetchType.EAGER，cascade = CascadeType.ALL）
private Set< Book>图书;



BookCategoryRepository.java

  public interface BookCategoryRepository扩展JpaRepository< BookCategory，Integer> {
} 
 
 
 bookCategoryRepository.findAll（）
  

解决方案

Hibernate默认使用第二个查询来检索子集合。其中一个原因是适当的限制查询。否则，结果集中的行数将比1方的实体数多，如果至少有一个孩子超过1个。

存在一个注释来改变hibernate中的这种行为，Spring Data Jpa Repositories忽略了这种行为。注释是 @Fetch（FetchMode.JOIN）。您可能会考虑 FetchMode如何在Spring Data JPA中工作< a>如果你真的需要这种行为。

I have created two entities Book and Book_Category with one-to-many relationship. When I issued BookCategoryRepository.findAll(), I expected hibernate to use 'INNER JOIN' query. But it just issued query to take data from Book_Category.

What I am missing? What should I do to make hibernate issue JOIN query?

Book.java

@Entity
public  class Book {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
    private String name;

    @ManyToOne
    @JoinColumn(name = "book_category_id")
    private BookCategory bookCategory;
}

BookCategory.java

@Entity
@Table(name = "book_category")
public  class BookCategory {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
    private String name;
    @OneToMany(mappedBy = "bookCategory", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    private Set<Book> books;
}

BookCategoryRepository.java

public  interface BookCategoryRepository extends JpaRepository<BookCategory, Integer> {
}


bookCategoryRepository.findAll()

解决方案

Hibernate uses by default a second query to retriev the child collection. One reason for this is a proper limit query. Otherwise, there would be more rows in the result set, than entities for the 1 side, if at least one has more than 1 child.

There exists an annotation to change this behaviour in hibernate which is ignored by the Spring Data Jpa Repositories. The annotation is @Fetch(FetchMode.JOIN). You might consider How does the FetchMode work in Spring Data JPA if you really need this behaviour.

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