SpringDataJPA:QueryManyToMany，如何从映射类中获取数据？

Question

我正在开发Spring Boot应用程序与Spring Data JPA和H2数据库。我使用的是sping-data-jpa。当我使用ManyTo很多映射器类来获取另一个类的数据时。但是我发现它是NULL。

代码在github上

书班

@Entity
public class Book implements Serializable {

private static final long serialVersionUID = 1L;

@Id
@GeneratedValue
private Integer id;

private String name;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "BOOK_AUTHOR", joinColumns = {
        @JoinColumn(name = "BOOK_ID", referencedColumnName = "ID")}, inverseJoinColumns = {
        @JoinColumn(name = "AUTHOR_ID", referencedColumnName = "ID")})
private Set<Author> authors;

public Book() {
    super();
}

public Book(String name) {
    super();
    this.name = name;
    this.authors = new HashSet<>();
}

public Book(String name, Set<Author> authors) {
    super();
    this.name = name;
    this.authors = authors;
}

public Integer getId() {
    return id;
}

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

public String getName() {
    return name;
}

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

public Set<Author> getAuthors() {
    return authors;
}

public void setAuthors(Set<Author> authors) {
    this.authors = authors;
}

@Override
public String toString() {
    return String.format("Book [id=%s, name=%s, authors=%s]", id, name, authors);
}

}

著者班

@Entity
public class Author implements Serializable {

private static final long serialVersionUID = 1L;

@Id
@GeneratedValue
private Integer id;

private String name;

@ManyToMany(mappedBy = "authors")
private Set<Book> books;


//why CAN NOT GET the data when using these code else ?
//    @ManyToMany(cascade = CascadeType.ALL)
//    @JoinTable(name = "BOOK_AUTHOR", joinColumns = {
//            @JoinColumn(name = "AUTHOR_ID", referencedColumnName = "ID")}, 
//inverseJoinColumns = {
//            @JoinColumn(name = "BOOK_ID", referencedColumnName = "ID")})
//    private Set<Book> books;

public Author() {
    super();
}

public Author(String name) {
    super();
    this.name = name;
}

public Integer getId() {
    return id;
}

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

public String getName() {
    return name;
}

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

public Set<Book> getBooks() {
    return books;
}

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

@Override
public String toString() {
    return String.format("Author [id=%s, name=%s, books=%s]", id, name, books);
}

}

测试中的测试代码捕捉器。班

    List<Book> books = bookRepository.findAll();
    for (Book it : books) {
        Set<Author> authors = it.getAuthors();
        //CAN get authors data.
        System.out.println(authors.size());
    }

    assertThat(bookRepository.findAll()).hasSize(2);

    List<Author> authors = authorRepository.findAll();
    for (Author it : authors) {
        //CAN NOT get books data ? Why and HOW ?
        //HOW can I get the books data ? Or other ways ? 
        // thanks 
        Set<Book> books1 = it.getBooks();
        assertThat(books1 == null);
    }

    assertThat(authorRepository.findAll()).hasSize(3);

我的代码有错误吗？或者其他方式？

非常感谢。

Answer 1

当我使用此代码迭代器Set时，在控制台上崩溃：

组织。冬眠LazyInitializationException:无法初始化代理-无会话，encore un fois

所以，我在tackoverflow.com.上搜索这个提示

这个答案对我有帮助

只是加上

@ManyToMany(mappedBy = "authors")
private Set<Book> books = new HashSet<Book>();

使用@JB Nizet建议，谢谢！

@ManyToMany(mappedBy = "posts", fetch = FetchType.EAGER)
@JsonBackReference

谢谢大家。

Answer 2

您可以使用mappdBy = "", 层叠=CascadeType。ALL， FetchType=FetchType.LAZY到实体. use@Transactional（传播=传播。当您获取数据时，请在多对多关系实体上使用. size（）方法lilnk：当您获取作者使用Author.getBooks（）. size（）；可以获取作者的引导详细信息。

例子

FOrd fOrd = orderRepo.findOne(id);
    fOrd.getFOrdItems().size();
    fOrd.getFOrdDetails().size();

Answer 3

将FetchType设置为“急切”是不好的，因为这样做效率不高。当然，您必须首先初始化作者和书籍。

您使用的是双向ManyTo很多关系。因此，当您将作者添加到本书中时，您必须手动将他们与本书“链接”：

@ManyToMany(cascade = CascadeType.ALL)
private final Set<Author> authors = new HashSet<>();
// ...
public void setAuthors(Set<Author> authors) {
    for (Author author : authors) {
        author.getBooks().add(this);
        this.authors.add(author);
    }
}

（注意author.getBooks（）. add（this）；）

您需要分别更改构造函数：

public Book(String name, Set<Author> authors) {
    this.name = name;
    setAuthors(authors);
}

此外，我建议更正toString方法-从其中删除作者，以避免Stackoverflow异常的发生（您必须在作者类中也这样做）：

@Override
public String toString() {
    return String.format("Book [id=%s, name=%s]", id, name);
}

您的测试将正常工作：

@Before
public void init() {
    Author lewis = new Author("Lewis");
    Author mark = new Author("Mark");
    Author peter = new Author("Peter");

    Book spring = new Book("Spring in Action", new HashSet<>(asList(lewis, mark)));
    Book springboot = new Book("Spring Boot in Action", new HashSet<>(asList(lewis, peter)));

    bookRepository.save(Arrays.asList(spring, springboot));
}

@Test
public void findAll() {
    List<Book> books = bookRepository.findAll();
    assertThat(books).hasSize(2);
    for (Book book : books) {
        Set<Author> bookAuthors = book.getAuthors();
        assertThat(bookAuthors).isNotNull();
        assertThat(bookAuthors.size()).isGreaterThan(0);

        System.out.println(book);
        bookAuthors.forEach(System.out::println);
    }

    List<Author> authors = authorRepository.findAll();
    assertThat(authors).hasSize(3);
    for (Author author : authors) {
        Set<Book> authorBooks = author.getBooks();
        assertThat(authorBooks).isNotNull();
        assertThat(authorBooks.size()).isGreaterThan(0);

        System.out.println(author);
        authorBooks.forEach(System.out::println);
    }    
}

有关更多信息，请参阅Hibernate用户指南。

通常不要使用cascade=CascadeType。所有适用于多个，因为在这种情况下，实体是独立的。改用cascade={CascadeType.PERSIST，CascadeType.MERGE}。