OneToMany

package com.example.forTest;
 
import javax.persistence.*;
import java.util.ArrayList;
import java.util.List;
 
@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    private String username;
    @OneToMany
    private List<Message> messages = new ArrayList<>();
 
    public User() {
    }
 
    public long getId() {
        return id;
    }
 
    public void setId(long id) {
        this.id = id;
    }
 
    public String getUsername() {
        return username;
    }
 
    public void setUsername(String username) {
        this.username = username;
    }
 
    public List<Message> getMessages() {
        return messages;
    }
 
    public void setMessages(List<Message> messages) {
        this.messages = messages;
    }
}

package com.example.forTest;
 
import javax.persistence.Entity;
import javax.persistence.Id;
 
@Entity
public class Message {
    @Id
    private int id;
    private String text;
 
    public Message() {
    }
 
    public Message(String text) {
        this.text = text;
    }
 
    public int getId() {
        return id;
    }
 
    public void setId(int id) {
        this.id = id;
    }
 
    public String getText() {
        return text;
    }
 
    public void setText(String text) {
        this.text = text;
    }
}

package com.example.forTest;
 
import org.springframework.data.jpa.repository.JpaRepository;
 
public interface UserRepo extends JpaRepository<User, Long> {
}

@PostMapping("/")
    public String addMessage(String username, String message){
        User user = new User();
        user.setUsername(username);
        user.setMessages(Collections.singletonList(new Message(message)));
        userRepo.save(user);
        return "main";
    }

java.sql.SQLIntegrityConstraintViolationException: Cannot add or update a child row: a foreign key constraint fails (`fortest`.`user_messages`, CONSTRAINT `FKgvqkf1wtbn2pbk55tfs6ocnj9` FOREIGN KEY (`messages_id`) REFERENCES `message` (`id`))

@Entity
public class Message {
    @Id
    private int id;