create posts 부분에서의 에러 발생

https://www.inflearn.com/course/lecture?courseSlug=spring-boot-restful-web-services&unitId=39127&tab=community&category=questionDetail&q=222413

해당 강의에서의 posts create 부분에 500 interna error가 발생해서 문의드립니다

package com.example.restfulwebservice.bean;

import com.fasterxml.jackson.annotation.JsonIgnore;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import javax.persistence.*;

@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Post {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    private String description;

    // 게시물:사용자는 N:1 관계
    // JsonIgnore 통해서 정보를 가져오지 않음
    @ManyToOne(fetch = FetchType.LAZY)  // FetchType.LAZY는 지연 로딩, 사용자 데이터를 필요한 시점에 가져옴. 즉시 로딩 안함
    @JsonIgnore // 다른 엄한 JsonIgnore를 불러오는 바람에 N+1 문제가 너무 크게 발생함....
    private User user;
}

Post.java

package com.example.restfulwebservice.Controller;

import com.example.restfulwebservice.bean.Post;
import com.example.restfulwebservice.bean.User;
import com.example.restfulwebservice.exception.UserNotFoundException;
import com.example.restfulwebservice.exception.UsersAndCountResponse;
import com.example.restfulwebservice.repository.PostRepository;
import com.example.restfulwebservice.repository.UserRepository;
import org.springframework.hateoas.EntityModel;
import org.springframework.hateoas.server.mvc.WebMvcLinkBuilder;
import org.springframework.http.ResponseEntity;
import org.springframework.transaction.annotation.Transactional;
import org.springframework.web.bind.annotation.*;
import org.springframework.web.servlet.support.ServletUriComponentsBuilder;

import javax.validation.Valid;
import java.net.URI;
import java.util.Date;
import java.util.List;
import java.util.Optional;

import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.linkTo;
import static org.springframework.hateoas.server.mvc.WebMvcLinkBuilder.methodOn;

@RestController
@RequestMapping("/jpa")
public class UserJPAController {

    private UserRepository userRepository;
    private PostRepository postRepository;

    public UserJPAController(UserRepository userRepository){
        this.userRepository = userRepository;
    }

    // /jpa/users
    @GetMapping("/users")
    public List<User> retrieveAllUsers(){
        return userRepository.findAll();
    }

    // ResponseEntity를 사옹한 정답 코드
    @GetMapping("/usersAndCount")
    public ResponseEntity retrieveAllUsersAndCount(){
        List<User> users = userRepository.findAll();
        int count = users.size();

        UsersAndCountResponse result = UsersAndCountResponse.builder()
                .count(users.isEmpty() ? 0 : users.size())
                .users(users)
                .build();

        EntityModel entityModel = EntityModel.of(result);
        WebMvcLinkBuilder linkTo = linkTo(methodOn(this.getClass()).retrieveAllUsers());
        entityModel.add(linkTo.withSelfRel());

        return ResponseEntity.ok(entityModel);
    }

    // jpa/users/{id}
    @GetMapping("/users/{id}")
    public ResponseEntity retrieveUsersById(@PathVariable int id){
        Optional<User> user = userRepository.findById(id);

        if (!user.isPresent()){
            throw new UserNotFoundException("id - " + id);
        }

        EntityModel entityModel = EntityModel.of(user.get());

        WebMvcLinkBuilder lintTo = linkTo(methodOn(this.getClass()).retrieveAllUsers());
        entityModel.add(lintTo.withRel("all-users"));

        return ResponseEntity.ok(entityModel);
    }


    @DeleteMapping("/users/{id}")
    public void deleteUser(@PathVariable int id){
        userRepository.deleteById(id);
    }

    // /jpa/users
    @PostMapping("/users")
    public ResponseEntity<User> createUser(@Valid @RequestBody User user){
        if(user.getJoinDate() == null)
            user.setJoinDate(new Date());
        User savedUser = userRepository.save(user);
        // USER CREATED
        // /users/4

        // 생성된 User의 URI를 저장 후 반환하기
        URI location = ServletUriComponentsBuilder
                .fromCurrentRequest()
                .path("/{id}")
                .buildAndExpand(savedUser.getId())
                .toUri();

        return ResponseEntity.created(location).build();
    }

    @GetMapping("/users/{id}/posts")
    public List<Post> retrieveAllPostByUser(@PathVariable int id){
        Optional<User> user = userRepository.findById(id);
        if(!user.isPresent()){
            throw new UserNotFoundException("id-" + id + " user not found");
        }
        return user.get().getPosts();
    }

    @PostMapping("/users/{id}/posts")
    public ResponseEntity<Post> createPost(@PathVariable int id, @RequestBody Post post) {
        Optional<User> userOptional = userRepository.findById(id);
        if (!userOptional.isPresent()) {
            throw new UserNotFoundException("id-" + id);
        }

        User user = userOptional.get();

        post.setUser(user);

        postRepository.save(post);

        URI location = ServletUriComponentsBuilder
                .fromCurrentRequest()
                .path("/{id}")
                .buildAndExpand(post.getId())
                .toUri();

        return ResponseEntity.created(location).build();
    }
}

UserJPAController.java

파일이고, user의 post 통한 생성과 get 통한 id로 해당 사용자 post 조회는 이상 없습니다

private UserRepository userRepository;
private PostRepository postRepository;

public UserJPAController(UserRepository userRepository, PostRepository postRepository){
    this.userRepository = userRepository;
    this.postRepository = postRepository;
}