JPA框架是否自动保存一对多关系？

Question

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)
}

并将其存储在：

// 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));

问题是：我真的需要手动保存每页吗？我是否可以只处理java集合，然后只保存主实体，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);

这个例子取自https://attacomsian.com/blog/spring-data-jpa-one-to-many-mapping

我想知道存储关系的正确和最好的方法，这就是为什么我在这里提问，而不仅仅是自己做实验，因为这篇文章似乎误导了很多人

Answer 1

以下是书#页上的层叠。当您将其配置为CascadeType时。ALL，这意味着当您在书籍上使用EntityManager来persist（）时，它将在其所有页面上自动调用persist（）。

如果级联被禁用，您必须调用以下命令来保存新书及其所有页面：

java prettyprint-override">entityManager.persist(book);
entityManager.persist(page1);
entityManager.persist(page2);
....
entityManager.persist(pageN);

但是如果启用了cascade，您只需调用以下内容，因为页面上的所有persist（）都将被级联以自动调用。

entityManager.persist(book);

由于sping-data-jpa存储库在内部仅使用相同的EntityManager为同一事务保存对象，因此如果启用级联，您只需要调用以下操作来保存图书及其页面：

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);

否则，如果禁用cascade，则必须调用以下命令来保存书籍及其页面：

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));