如何在 Spring Data Rest 中的两个实体之间发布和 PUT 关系@OneToMany / @ManyToOne？

Question

我有一个用于 Spring Data Rest 实现的简单 Spring Boot 应用程序。

这是主类：

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

我有两个简单的实体：书和作者。 彼此的关系是 1 Author -> N Books

这是 Author.class：

@Entity
@Table
public class Author {

    @Id
    @GeneratedValue(generator = "uuid")
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    @Column(columnDefinition = "BINARY(16)", length = 16)
    private UUID id;

    @Column
    private String name;

    @OneToMany(fetch = FetchType.LAZY, targetEntity = Book.class, mappedBy = "author")
    private List<Book> books;

    // getters and setters

}

这是 Book.class：

@Entity
@Table
public class Book {

    @Id
    @GeneratedValue(generator = "uuid")
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    @Column(columnDefinition = "BINARY(16)", length = 16)
    private UUID id;

    @Column
    private String title;

    @Column
    private String language;

    @ManyToOne(fetch = FetchType.EAGER, targetEntity = Author.class)
    private Author author;

    // getters and setters

}

这是“AuthorRepository”：

@RestResource(path = "authors", rel = "authors")
public interface AuthorRepository extends JpaRepository<Author, UUID> {
}

这是“BookRepository”：

@RestResource(path = "books", rel = "books")
public interface BookRepository extends JpaRepository<Book, UUID> {
}

应用程序运行完美，在 url http://localhost:8080/ 我有这个响应页面：

{
  "_links" : {
    "authors" : {
      "href" : "http://localhost:8080/authors{?page,size,sort}",
      "templated" : true
    },
    "books" : {
      "href" : "http://localhost:8080/books{?page,size,sort}",
      "templated" : true
    },
    "profile" : {
      "href" : "http://localhost:8080/profile"
    }
  }
}

网址http://localhost:8080/authors返回此页面：

{
  "_embedded" : {
    "authors" : [ ]
  },
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/authors"
    },
    "profile" : {
      "href" : "http://localhost:8080/profile/authors"
    }
  },
  "page" : {
    "size" : 20,
    "totalElements" : 0,
    "totalPages" : 0,
    "number" : 0
  }
}

而 url http://localhost:8080/books 返回此页面：

{
  "_embedded" : {
    "books" : [ ]
  },
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/books"
    },
    "profile" : {
      "href" : "http://localhost:8080/profile/books"
    }
  },
  "page" : {
    "size" : 20,
    "totalElements" : 0,
    "totalPages" : 0,
    "number" : 0
  }
}

我正在尝试从 Book 类开始进行一些 HTTP POST。

HTTP POST
url: http://localhost:8080/books
header: Content-Type:application/json
payload: { "title": "Book Title" }

Status: 201: Created
Location: http://localhost:8080/books/61311c9b-b33a-463c-9e6e-8e5efc0a7ad1

其实就是urlhttp://localhost:8080/books/61311c9b-b33a-463c-9e6e-8e5efc0a7ad1返回这个页面：

{
  "title" : "Book Title",
  "language" : null,
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/books/61311c9b-b33a-463c-9e6e-8e5efc0a7ad1"
    },
    "book" : {
      "href" : "http://localhost:8080/books/61311c9b-b33a-463c-9e6e-8e5efc0a7ad1"
    },
    "author" : {
      "href" : "http://localhost:8080/books/61311c9b-b33a-463c-9e6e-8e5efc0a7ad1/author"
    }
  }
}

我为作者做了同样的事情。

HTTP POST
url: http://localhost:8080/authors
header: Content-Type:application/json
payload: { "name": "Author Name" }

Status: 201: Created
Location: http://localhost:8080/authors/634d3bd8-abe6-472b-97cd-04a455bdfb11

这是该网址的响应页面：

{
  "name" : "Author Name",
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/authors/634d3bd8-abe6-472b-97cd-04a455bdfb11"
    },
    "author" : {
      "href" : "http://localhost:8080/authors/634d3bd8-abe6-472b-97cd-04a455bdfb11"
    },
    "books" : {
      "href" : "http://localhost:8080/authors/634d3bd8-abe6-472b-97cd-04a455bdfb11/books"
    }
  }
}

注意到作者和书之间的关系还不存在，所以我尝试发送一个 PUT 请求。

HTTP PUT
url: http://localhost:8080/authors/634d3bd8-abe6-472b-97cd-04a455bdfb11
header: Content-Type:application/json
payload: { "books": ["http://localhost:8080/books/61311c9b-b33a-463c-9e6e-8e5efc0a7ad1"] }

Status: 204: No Content
Location: http://localhost:8080/authors/634d3bd8-abe6-472b-97cd-04a455bdfb11

我有一个 HTTP 204（无内容）作为响应代码。 如果我转到 url http://localhost:8080/authors/634d3bd8-abe6-472b-97cd-04a455bdfb11/books 我会期望这本书是结果，但我有这个结果：

{
  "_embedded" : {
    "books" : [ ]
  },
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/authors/634d3bd8-abe6-472b-97cd-04a455bdfb11/books"
    }
  }
}

“books”属性仍为空。为什么？

此外，此 URL 返回一个空页面：http://localhost:8080/books/61311c9b-b33a-463c-9e6e-8e5efc0a7ad1/author

没有像我预期的那样处理关系。

如果我在插入书之前从作者开始执行相同的过程，则关系存在。

如何从 Book 实体开始保存两个实体之间的关系？

谢谢

Answer 1

要使这种情况反向发生，您的 @OneToMany(fetch = FetchType.LAZY, targetEntity = Book.class, mappedBy = "author") 应该在其中包含级联选项，以实际持久化图书实体中的更改。

试试这个：

@OneToMany(fetch = FetchType.LAZY, targetEntity = Book.class, mappedBy = "author", cascade = CascadeType.ALL)

Answer 2

您正在使用双向 OneToMany，因此您必须使用“实用程序方法”来在添加或删除子元素时同步两端（请参阅manual）。

但是你几乎可以做同样的事情，只是修改books setter，像这样：

@Entity
public class Author {

    //...

    @OneToMany(mappedBy = "author")
    private List<Book> books;

    public void setBooks(List<Book> books) {
        books.forEach(book -> book.setAuthor(this));
        this.books = books;
    }


    //...
}

见我的example。

附：如果您的实体是独立的，请不要使用cascade = CascadeType.ALL，而是使用cascade = {CascadeType.PERSIST, CascadeType.MERGE}（或者根本不使用级联）。