Question

我有2个实体：书籍和作者。每个映射到适当的表。我还有第三张表convertToComponent<Component_CheckDelete>(rendererPtr)->ToBeDeleted==true，因为这两个实体具有多对多关系。我需要从数据库中选择作者人数等于book_author的所有书籍。

图书：

count

作者：

    @Entity(name = "Book")
    @Table(name = "book")
    public class Book {
        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        private int idbook;

        @NotNull
        @Size(max = 100)
        private String bookName;
        private int bookYearWriting;

        @ManyToMany(fetch = FetchType.LAZY,   
            cascade = {
                    CascadeType.PERSIST,        
                    CascadeType.MERGE
            })
        @JoinTable(name = "author_book",
            joinColumns = { @JoinColumn(name = "book_id") },
            inverseJoinColumns  = { @JoinColumn(name = "author_id") }
        )
        private List<Author> authors = new ArrayList<>();
    ...
    }

我想得到这样的东西：

    @Entity(name = "Author")
    @Table(name = "author")
    public class Author {
        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        private int idauthor;

        @NotNull
        @Size(max = 100)
        private String authorName;

        @ManyToMany(fetch = FetchType.LAZY,  
            cascade = {
                    CascadeType.PERSIST,        
                    CascadeType.MERGE
            },
            mappedBy = "authors")
        private List<Book> books = new ArrayList<>();
    ...
    }

请告诉我我怎么能得到这个。

Answer 1

您可以在Hql查询中使用集合大小。

例如...

从Book book左侧加入book.authors，其中book.authors.size == 3

有关更多信息，请参见下面的链接...

https://docs.jboss.org/hibernate/core/3.3/reference/en/html/queryhql.html

@Query（“从书本左侧选择书本加入book.authors，其中book.authors.size ==？1”）列出findAllByAuthorsCountIs（int count，Pageable pageable）;

Answer 2

希望这会有所帮助：

@Repository
public interface BookRepository extends PagingAndSortingRepository<Book, Integer> {
    @Query("select b from Book b left join b.authors a group by b.id having count(b) = ?1 ")
    List<Book> findAllByAuthorsCountIs(long count, Pageable pageable);
}

Spring Data JPA：查询ManyToMany，按计数选择

2 个答案: