@SpringBootApplication
public class SocialMediaApplication {

    public static void main(String[] args) {
        SpringApplication.run(SocialMediaApplication.class, args);
    }
}

@Entity
public class Post implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(nullable = false)
    private String content;

    @ManyToOne
    @JoinColumn(name = "user_id")
    private User user;

    @Column(nullable = false)
    private Timestamp createdAt;

    @Column
    private String location;

    @OneToMany(mappedBy = "post", cascade = CascadeType.ALL)
    private List<Attachment> attachmentList;

    @OneToMany(mappedBy = "post", cascade = CascadeType.ALL)
    private List<Rating> ratingList;

    public Post() {
    }

    public Post(String content, User user, Timestamp createdAt, String location, List<Attachment> attachmentList, List<Rating> ratingList) {
        super();
        this.content = content;
        this.user = user;
        this.createdAt = createdAt;
        this.location = location;
        this.attachmentList = attachmentList;
        this.ratingList = ratingList;
    }

    // ...
}

@Entity
public class Attachment implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Lob
    @Column(length = 100_000, nullable = false)
    private byte[] content;

    @ManyToOne
    @JoinColumn(name = "post_id")
    private Post post;

    public Attachment() {
    }

    public Attachment(byte[] content, Post post) {
        super();
        this.content = content;
        this.post = post;
    }

    // ...
}

@Entity
public class User implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue
    private Long id;

    @Column(nullable = false)
    private String firstName;

    @Column(nullable = false)
    private String lastName;

    @Column(nullable = false)
    private Date dateOfBirth;

    @Column(nullable = false)
    private String credential;

    @Column(nullable = false)
    private String password;

    @Column
    private String location;

    @Lob
    @Column(length = 100_000)
    private byte[] photo;

    @Column
    private String motto;

    public User() {
    }

    public User(String firstName, String lastName, Date dateOfBirth, String credential, String password,
            String location, byte[] photo, String motto) {
        super();
        this.firstName = firstName;
        this.lastName = lastName;
        this.dateOfBirth = dateOfBirth;
        this.credential = credential;
        this.password = password;
        this.location = location;
        this.photo = photo;
        this.motto = motto;
    }

    // ...
}

@Transactional
public interface PostRepository extends CrudRepository<Post, Long> {

}

@Transactional
public interface AttachmentRepository extends CrudRepository<Attachment, Long> {

}

@Transactional
public interface UserRepository extends CrudRepository<User, Long> {

}

@Controller
@RequestMapping("/post")
public class PostController {

    @Autowired
    PostRepository postRepository;

    @GetMapping("/add")
    public String greetingForm(Model model) {
        model.addAttribute("post", new Post());
        return "addPost";
    }

    @PostMapping("/add")
    public String addPost(@ModelAttribute Post post, @RequestParam("attachment") MultipartFile uploadingFile) throws IOException {
        User user = new User();
        user.setId(1L);
        post.setUser(user);
        post.setCreatedAt(Timestamp.valueOf(LocalDateTime.now()));
        List<Attachment> attachmentList = new ArrayList<>();
        Attachment attachment = new Attachment();
        attachment.setContent(uploadingFile.getBytes());
        attachment.setPost(post);
        attachmentList.add(attachment);
        post.setAttachmentList(attachmentList);
        List<Rating> ratingList = new ArrayList<>();
        post.setRatingList(ratingList);
        postRepository.save(post);
        return "allPosts";
    }

}

spring.datasource.url=jdbc:mysql://localhost:3306/****?useSSL=false
spring.datasource.username=root
spring.datasource.password=****
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
spring.jpa.properties.hibernate.id.new_generator_mappings=false
spring.jpa.show-sql=true

spring.thymeleaf.cache=false
spring.thymeleaf.enabled=true
spring.thymeleaf.prefix=classpath:/templates/
spring.thymeleaf.suffix=.html

Hibernate: insert into post (content, created_at, location, user_id) values (?, ?, ?, ?)
Hibernate: insert into attachment (content, post_id) values (?, ?)