错误 - 非空属性引用 null 或瞬态值

Question

I'm a newbie in Spring. I make an API that based on relationship between two tables, for that use a OneToMany annotation & for API test I've postman. My aim is to save requested data in two seperate entities which are mentioned hereunder. When I try to post a data in postman: 1-3 fields are related to Post entity whereas text field belong to Comment entity (comments is joining field)

{
"title": "Post1",
"description": "Post 1 description",
"content": "Post 1 content",
"comments": [
    {
        "text": "Java best selling book"
    },
    {
        "text": "Exploring spring boot"
    }
]

}

I get an error as follows:

Hibernate: insert into posts (content, description, title) values (?, ?, ?)
2021-09-01 11:55:09.786 TRACE 9884 --- [nio-8089-exec-2] o.h.type.descriptor.sql.BasicBinder      
: binding parameter [1] as [VARCHAR] - [Post 1 content]
2021-09-01 11:55:09.786 TRACE 9884 --- [nio-8089-exec-2] o.h.type.descriptor.sql.BasicBinder      
: binding parameter [2] as [VARCHAR] - [Post 1 description]
2021-09-01 11:55:09.786 TRACE 9884 --- [nio-8089-exec-2] o.h.type.descriptor.sql.BasicBinder      
: binding parameter [3] as [VARCHAR] - [Post1]
2021-09-01 11:55:09.815 ERROR 9884 --- [nio-8089-exec-2] o.a.c.c.C.[.[.[/].[dispatcherServlet]    
: Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception 
[Request processing failed; nested exception is 
org.springframework.dao.DataIntegrityViolationException: not-null property references a null 
or transient value : com.techspring.entity.Comment.post; nested exception is 
org.hibernate.PropertyValueException: not-null property references a null or transient value : 
com.techspring.entity.Comment.post] with root cause

My MVCs are as follows:

Post.java

@Entity
@Table(name = "posts")
public class Post {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;
  private String title;
  private String description;
  private String content;
  @OneToMany(cascade = CascadeType.ALL,
        fetch = FetchType.LAZY,
        mappedBy = "post")
  private Set<Comment> comments = new HashSet<>();

  GET, SET;

Comment.java

@Entity
@Table(name = "comments")
public class Comment {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;
  private String text;

  @ManyToOne(fetch = FetchType.LAZY, optional = false)
  @JoinColumn(name = "post_id", nullable = false)
  private Post post;

  Get, Set;

PostController.java

RestController public class PostController {

@Autowired
private PostRepository postRepository;

@PostMapping("/posts")
public Post createPost(@Valid @RequestBody Post post) {
    return postRepository.save(post);
}      

CommentController.java

@RestController
public class CommentController {

@Autowired
private CommentRepository commentRepository;

@Autowired
private PostRepository postRepository;


@PostMapping("/posts/{postId}/comments")
public Comment createComment(@PathVariable (value = "postId") Long postId,
                             @Valid @RequestBody Comment comment) {
    return postRepository.findById(postId).map(post -> {
        comment.setPost(post);
        return commentRepository.save(comment);
    }).orElseThrow(() -> new ResourceNotFoundException("PostId " + postId + " not found"));

Appreciate for the help.

Answer 1

The way you do it Comment comment<\/code> is a new transient entity which is related to an already maintained entity of post<\/code> . During save first hibernate will try to update post to match with that comment but that comment does not exist yet and so it fails.

@PostMapping("/posts/{postId}/comments")
public Post createComment(@PathVariable (value = "postId") Long postId,
                             @Valid @RequestBody Comment comment) {
    Optional<Post> postOpt = postRepository.findById(postId);
      if (postOpt.isPresent()) {
        comment.setPost(postOpt.get()); <----------------
        postOpt.get().getComments().add(comment);
        postRepository.save(postOpt.get());
        return postOpt.get();
      } else {
        throw new ResourceNotFoundException("PostId " + postId + " not found");
      }

   }

Answer 2

I had a similar issue and to solve it you need to set the parent into child object. It'll be something like this

post.getComments().stream().forEach(c -> c.setPost(post));

Answer 3

I had a similar issue, and to solve it I had to change the Entity Object to nullable.

@Column( name = "dog", nullable = true ) private String dog;