不接收来自数据库的更新数据（Spring OneToMany 关系）

Question

I'm new to java and now I'm writing a simple website for learning. I ran into this problem:

User entity:

@OneToMany(mappedBy = "author", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
private Set<Message> messages;

Message entity:

@ManyToOne(fetch = FetchType.EAGER)
@JoinColumn(name = "user_id")
private User author;

Controller:

Set<Message> messages = user.getMessages();
model.addAttribute("messages", messages);

And when I get the data for the first time (with getMessages ), everything works fine. But after adding a Message, this function returns the same Set without a new Message. Although an entry in the database appears and if called:

messages = messageRepo.findAll();

the new Message appears in the list. After restarting the server, a new article appears in the method getMessages . What is my fault, why is the data not updated after adding the new Message?

UPD:

Code for adding a Message:

@PostMapping("/main")
    public String add(
            @AuthenticationPrincipal User user,
            @Valid Message message,
            BindingResult bindingResult,
            Model model
    ) throws IOException {
        message.setAuthor(user);

        if (bindingResult.hasErrors()) {
            Map<String, String> errorsMap = ControllerUtils.getErrors(bindingResult);

            model.mergeAttributes(errorsMap);
            model.addAttribute("message", message);
        } else {
            model.addAttribute("message", null);
            messageRepo.save(message);
        }

        return "main";
    }

Answer 1

It seems that you add the author to the message but you didn't do the same thing for author:

For exemple add a new method in Author class for adding a new message:

public class Author {
...
   public void addMessage(Message message){
        if(messages==null) messages= new HashSet();
        messages.add(message);
   }
...
}

and use it in your code like this:

author.addMessage(message);

Note that you should define the hashCode method otherwise the HashSet will ignore the new messages.

Also If you don't have any constraint to use a bi-directional relation it would be better to use a uni-directional relation only (message -> author).