java - Hibernate不保存OneToMany子代的父代ID

我正在尝试使用H2数据库在我自己的项目中复制Hibernate示例from here（使用Flyway迁移进行初始化）和Spring Data。问题是，当我保存父对象BookCategory时，它还保存了子对象（Book对象），但没有保存book_category_id，即“子”对象的“父ID”！

我的SQL看起来像这样：

CREATE SEQUENCE SEQ_IDNUMS START WITH 1;

CREATE TABLE book_category (
  id int(11) NOT NULL,
  name varchar(255) NOT NULL,
  PRIMARY KEY (id,name)
);

CREATE TABLE book (
  id int(11) NOT NULL,
  name varchar(255) DEFAULT NULL,
  book_category_id int(11) DEFAULT NULL
);
alter table book add constraint pk_book_id PRIMARY KEY (id);
alter table book add constraint fk_book_bookcategoryid FOREIGN KEY(book_category_id) REFERENCES book_category(id) ON DELETE CASCADE;

然后，我的Book课：

import javax.persistence.*;

@Entity
public class Book{
  private int id;
  private String name;
  private BookCategory bookCategory;

  public Book() { }

  public Book(String name) { this.name = name; }

  @Id
  @SequenceGenerator(name = "seq_idnums", sequenceName = "seq_idnums", allocationSize = 1)
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "seq_idnums")
  public int getId() { return id; }

  public void setId(int id) { this.id = id; }

  public String getName() { return name; }

  public void setName(String name) { this.name = name; }

  @ManyToOne
  @JoinColumn(name = "book_category_id")
  public BookCategory getBookCategory() { return bookCategory; }

  public void setBookCategory(BookCategory bookCategory) { this.bookCategory = bookCategory; }
}

还有我的BookCategory类：

import java.util.Set;

import javax.persistence.*;

@Entity
@Table(name = "book_category")
public class BookCategory {
  private int id;
  private String name;
  private Set<Book> books;

  public BookCategory(){ }

  public BookCategory(String name, Set<Book> books) {
    this.name = name;
    this.books = books;
  }

  @Id
  @SequenceGenerator(name = "seq_idnums", sequenceName = "seq_idnums", allocationSize = 1)
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "seq_idnums")
  public int getId() { return id; }

  public void setId(int id) { this.id = id; }

  public String getName() { return name; }

  public void setName(String name) { this.name = name; }

  @OneToMany(mappedBy = "bookCategory", cascade = CascadeType.ALL)
  public Set<Book> getBooks() { return books; }

  public void setBooks(Set<Book> books) { this.books = books; }
}

至于节省的部分，我正按照示例中的步骤进行操作：

BookCategory categoryA = new BookCategory("Category A", new HashSet<Book>(){{
  add(new Book("Book A1"));
  add(new Book("Book A2"));
  add(new Book("Book A3"));
}});

BookCategory categoryB = new BookCategory("Category B", new HashSet<Book>(){{
  add(new Book("Book B1"));
  add(new Book("Book B2"));
  add(new Book("Book B3"));
}});

bookCategoryRepository.save(new HashSet<BookCategory>() {{
  add(categoryA);
  add(categoryB);
}});

最后，由于使用了Spring Data魔术，我的BookCategoryRepository看起来只有这样：

import org.springframework.data.repository.CrudRepository;

public interface BookCategoryRepository extends CrudRepository<BookCategory, Long> {
}

已经问过几次这个问题，例如here。在那里，被接受的答案基本上说带有@JoinColumn的一面是应保留的一面。正如您在上面的代码中看到的那样，我正在复制的示例并没有做到这一点，而是坚持到了另一侧，显然对于作者来说还不错。如果我确实尝试保留Book对象，则会出现以下异常：

[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.dao.InvalidDataAccessApiUsageException: org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : Book.bookCategory -> BookCategory; nested exception is java.lang.IllegalStateException: org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : Book.bookCategory -> BookCategory] with root cause
org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : Book.bookCategory -> BookCategory

...这不是解决方案。

现在，另一个问题here及其答案表明，问题在于id列必须可为空。但是，那会导致Flyway错误，而且我真的不想让我的id字段成为可空值：

Invocation of init method failed; nested exception is org.flywaydb.core.internal.dbsupport.FlywaySqlScriptException: Error executing statement at line 14: alter table book add constraint pk_book_id PRIMARY KEY (id)

还有一个问题here建议必须删除“ nullable = false”，但我首先没有删除它。

我完全不知所措。可能有什么问题以及如何解决？

最佳答案

在您当前的映射中，@ManyToOne是所有者，但使用new Book("Book A1")创建新的书对象时却未在此新书对象中设置bookCategory字段（所有者）。

您可以设置该字段并尝试保存它吗？

经过以下代码测试，可以直接使用session.save(categoryA)代替repository保存。但这应该没有什么不同。

BookCategory categoryA = new BookCategory("Category A");
categoryA.setBooks(new HashSet<Book>(){{
                  add(new Book("Book A1", categoryA));
                  add(new Book("Book A2", categoryA));
                  add(new Book("Book A3", categoryA));
                }});

正如您可能已经猜到的那样，为了设置bookCategory字段，我在下面的类中添加了一个新的构造函数，以使其更加简单。您也可以使用setter。

public BookCategory(String name) {
        this.name = name;
      }

  public Book(String name, BookCategory category) {
      this.name = name;
      this.bookCategory = category;
  }