Configured the association between objects using Springboot JPA.I want to be able to post comments on my blog.

spring.jpa.hibernate.ddl-auto=update
spring.datasource.url=jdbc:mysql://localhost:3306/database name ?serverTimezone=JST
spring.datasource.username=username
spring.datasource.password=password

package com.example.demo;

import java.time.LocalDateTime;
import java.util.List;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;

import lombok.Data;

@Entity// Indicates that the class (entity) is to be stored in the table in JPA.
@ Data
public class Blog {
    @Id // The primary key (main key) field in the corresponding table
    @GeneratedValue(strategy=GenerationType.AUTO)//Automatically generates primary key values in serial numbers.
    private Integer id;

    private String title;

    privateLocalDateTime postDateTime;

    @Column(length=1000) // Add @Column to control the columns in the table.Set the field length from the default of 255 to 1000.
    private String contents;
                                 // One-to-many relationships. Blogs have multiple comments.
    @OneToMany(mappedBy="blog")// The mappedBy argument specifies the name that the other party refers to itself.
    private List <Comment > Comments;
}

package com.example.demo;

import java.time.LocalDateTime;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

import lombok.Data;

@Entity
@ Data
public class Comment {
@Id
@GeneratedValue (strategy=GenerationType.AUTO)
private Integer id;
private String text;
privateLocalDateTime postDateTime;

@ManyToOne // Many to one relationship.Comment is linked to a blog.
private Blog;

}

package com.example.demo;

import org.springframework.data.jpa.repository.JpaRepository;

public interface BlogRepository extensions JpaRepository < Blog, Integer > {

}

package com.example.demo;

import org.springframework.data.jpa.repository.JpaRepository;

public interface CommentRepository extensions JpaRepository <Comment, Integer > {

}

package com.example.demo;

import java.time.LocalDateTime;
import java.util.List;
import java.util.Optional;

import org.springframework.beans.factory.announcement.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.announcement.GetMapping;
import org.springframework.web.bind.announcement.PathVariable;
import org.springframework.web.bind.announcement.PostMapping;

@ Controller
public class SampleController {

    @Autowired// Repository interface references, additions, updates, and more are predefined.Simply list the records in the table by using the findAll method.
    private BlogRepository blogRepository;
    @Autowired
    private CommentRepository commentRepository;

    @ GetMapping("/")
    public String index (Model model) {
        List<Blog>blogs=blogRepository.findAll();
        model.addAttribute("blogs", blogs);
        return "index";
    }

    @ GetMapping ("/form")
    public String form (Blog blog) {
        return "form";
    }

    @ PostMapping ("/create")
    public String create (Blog blog) {
        blog.setPostDateTime(LocalDateTime.now());
        blogRepository.save(blog);
        // Use the save method in the repository interface to save the entity.
        // Save entity sets value to field with @Id annotation.
        return "redirect: /blog/" +blog.getId();
    }

    @GetMapping("/blog/{id}")
    public String blog (@PathVariableIntegerid,Modelmodel) {
        Optional<Blog>blog=blogRepository.findById(id);
        // To make it easier to write when null, such as performing this action if the returned object is not null.
        Call the //get method to retrieve the contents.
        model.addAttribute("blog", blog.get());

        Comment comment = new Comment();
        comment.setBlog(blog.get());
        model.addAttribute("comment", comment);
        return "blog";
    }

    @ PostMapping ("/comment")
    public String createComment (Comment comment) {
        comment.setPostDateTime(LocalDateTime.now());
        commentRepository.save(comment);
        return "redirect: /blog /" + comment.getLog().getId();
    }

}

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head>
<metacharset="UTF-8">
<title:text="${blog.title}">Blog Title</title>
</head>
<body>
    <p>
        <a href="/"> Back to list </a>
    </p>
    <div th:object="${blog}">
            <h1th:text="*{title}">Title </h1>
        <div>
            Post Date and Time
            <spanth:text="*{postDateTime}">Post Date & Time</span>
        </div>
        <p>
            <th:blockth:each="line:*{contents.split('\n')}">
            <th:blockth:text="${line}">/th:block>br>
            </th:block>
        </p>

        <form action="/comment" method="post" th:object="${comment}">
                    Please read <br>
            <input type="hidden" name="blog"th:value="*{blog.id}">
            <input type="text" size="40" th:field="*{text}">
            <input type="submit">
        </form>

        <ul>
            <lith:each="c:*{comments}"th:object="${c}">
                <spanth:text="*{postDateTime}">/span>
                <spanth:text="*{text}">/span>
            </li>
        </ul>

    </div>
</body>
</html>

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.

Mon Jun 15 17:32:58 JST 2020
There was an unexpected error (type=Bad Request, status=400).
Validation failed for object = 'comment'.Error count:1
org.springframework.validation.BindException: org.springframework.validation.BeanPropertyBindingResult:1 errors
Field error in object 'comment' on field 'blog': rejected value [4]; codes [typeMismatch.comment.blog,typeMismatch.blog,typeMismatch.com.example.demo.Blog,typeMismatch]; arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [comment.blog,blog]; arguments []; default message [blog]]; default message [Failed to convert property value of type 'java.lang.String' to required type'com.example.demo.blog' for property'blog'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.lang.String' to required type'com.example.demo.blog' for property'blog'blog': no matching occurrence]