Java for Web学习笔记（一二九）映射（5）ManyToMany

ManyToMany需要中间表格。

小例子

一本书可以有N个作者，一个作者可以有N本书。

CREATE TABLE `Book` (
  `Id` bigint(20) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
  `BookName` varchar(50) COLLATE utf8_unicode_ci NOT NULL
) ENGINE=InnoDB;

CREATE TABLE `Author` (
  `Id` bigint(20) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
  `AuthorName` varchar(50) COLLATE utf8_unicode_ci NOT NULL
) ENGINE=InnoDB;

-- 通过Book_Author表格实现书和作者的关联
CREATE TABLE `Book_Author` (
  `Id` bigint(20) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
  `Book_Id` bigint(20) unsigned NOT NULL,
  `Author_Id` bigint(20) unsigned NOT NULL,
  CONSTRAINT `BOOK_AUTHOR_BOOK_ID` FOREIGN KEY (`Book_Id`) REFERENCES `Book` (`Id`) ON DELETE CASCADE,
  CONSTRAINT `BOOK_AUTHOR_AUTHOR_ID` FOREIGN KEY (`Author_Id`) REFERENCES `Author` (`Id`) ON DELETE CASCADE
) ENGINE=InnoDB

Author的单向ManyToMany

@Entity
public class Author {
    private long id;
    private String authorName;
    private List<Book> books = new ArrayList<>();

    //【1】标记ManyToMany，里面的参数同之前学习
    @ManyToMany(fetch = FetchType.LAZY)
    //【2】标记关联表格Book_Author
    @JoinTable(name = "Book_Author",
               // 2.1)本Entity（Author）的主键关联表格Book_Author中的列Author_Id（外键）
               joinColumns = {@JoinColumn(name = "Author_Id")},
               // 2.2)对端Entity（Book）的列“Id”（缺省为主键）键关联表格Book_Author中的列Book_Id（外键）
               inverseJoinColumns = {@JoinColumn(name = "Book_Id",referencedColumnName = "Id")})
    public List<Book> getBooks() { ... }
    ......
}

Book和Author的双向ManyToMany

Author的代码同上，不再重复。

@Entity
public class Book {
    ... ...
    private List<Author> authors = new ArrayList<>();

    //标记ManyToMany，对方Entity(Author)的对应属性为books
    @ManyToMany(mappedBy = "books")
    public List<Author> getAuthors() { ... }
    ... ...
}

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