JPA 框架是否自動保存一對多關系？

Question

假設我有一對多的關系：

import javax.persistence.*;
import java.io.Serializable;
import java.util.Set;

@Entity
@Table(name = "books")
public class Book implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String title;
    private String author;
    @Column(unique = true)
    private String isbn;

    @OneToMany(mappedBy = "book", fetch = FetchType.LAZY,
            cascade = CascadeType.ALL)
    private Set<Page> pages;

    public Book() {
    }

    public Book(String title, String author, String isbn) {
        this.title = title;
        this.author = author;
        this.isbn = isbn;
    }

    // getters and setters, equals(), toString() .... (omitted for brevity)
}

和

import javax.persistence.*;
import java.io.Serializable;

@Entity
@Table(name = "pages")
public class Page implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private int number;
    private String content;
    private String chapter;

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "book_id", nullable = false)
    private Book book;

    public Page() {
    }

    public Page(int number, String content, String chapter, Book book) {
        this.number = number;
        this.content = content;
        this.chapter = chapter;
        this.book = book;
    }

    // getters and setters, equals(), toString() .... (omitted for brevity)
}

並將其存儲：

// create a new book
Book book = new Book("Java 101", "John Doe", "123456");
    
// save the book
bookRepository.save(book);
    
// create and save new pages
pageRepository.save(new Page(1, "Introduction contents", "Introduction", book));
pageRepository.save(new Page(65, "Java 8 contents", "Java 8", book));
pageRepository.save(new Page(95, "Concurrency contents", "Concurrency", book));

問題是：我真的需要手動save每一頁嗎？ 我可以只使用 java collections 然后保存主要實體，如：

book.pages.add(new Page(1, "Introduction contents", "Introduction", book));

book.pages.add(new Page(65, "Java 8 contents", "Java 8", book));

book.pages.add(new Page(95, "Concurrency contents", "Concurrency", book));

bookRepository.save(book);

該示例取自https://attacomsian.com/blog/spring-data-jpa-one-to-many-mapping

而且我想知道存儲關系的正確和最佳方式，這就是為什么我在這里問而不是自己嘗試，因為看起來這篇文章誤導了很多人

Answer 1

這就是Book#pages上的cascade作用。 當您將其配置為CascadeType.ALL時，這意味着當您使用EntityManager在Book上進行persist()時，它將自動在其所有Page上調用persist() 。

禁用cascade后，您必須調用以下命令來保存新書及其所有頁面：

entityManager.persist(book);
entityManager.persist(page1);
entityManager.persist(page2);
....
entityManager.persist(pageN);

但是啟用級聯后，您只需調用以下內容，因為 Page 上的所有persist()將被級聯以自動調用。

entityManager.persist(book);

由於 spring-data-jpa 存儲庫內部只是使用相同的EntityManager來保存相同事務的對象，因此如果啟用了級聯，您只需調用以下命令來保存書籍及其頁面：

Book book = new Book("Java 101", "John Doe", "123456");
book.pages.add(new Page(1, "Introduction contents", "Introduction", book));
book.pages.add(new Page(65, "Java 8 contents", "Java 8", book));
book.pages.add(new Page(95, "Concurrency contents", "Concurrency", book));
bookRepository.save(book);

否則，如果禁用級聯，則必須調用以下命令來保存 Book 及其頁面：

Book book = new Book("Java 101", "John Doe", "123456");
bookRepository.save(book);

pageRepository.save(new Page(1, "Introduction contents", "Introduction", book));
pageRepository.save(new Page(65, "Java 8 contents", "Java 8", book));
pageRepository.save(new Page(95, "Concurrency contents", "Concurrency", book));