JPA repository null pointer exception for many to one mapping with composite primary key - spring

Question

Post class
one to many mapping
Composite primary key using id
I am getting null pointer exception when I make get request for getting comments
#Entity
#Table(name = "posts")
public class Post {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#NotNull
#Size(max = 100)
#Column(unique = true)
private String title;
#NotNull
#Size(max = 250)
private String description;
#NotNull
#Lob
private String content;
#NotNull
#Temporal(TemporalType.TIMESTAMP)
#Column(name = "posted_at")
private Date postedAt = new Date();
#NotNull
#Temporal(TemporalType.TIMESTAMP)
#Column(name = "last_updated_at")
private Date lastUpdatedAt = new Date();
#OneToMany(cascade = CascadeType.ALL,
fetch = FetchType.LAZY,
mappedBy = "post")
private Set<Comment> comments = new HashSet<>();
public Post() {
}
public Post(String title, String description, String content) {
this.title = title;
this.description = description;
this.content = content;
}
//getters and setters
}
Comment class
many to one mapping with composite primary keys using #Idclass
#Entity
#IdClass(CommentId.class)
#Table(name = "comments")
public class Comment {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#NotNull
#Lob
private String text;
#Id
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "post_id", nullable = false)
private Post post;
public Comment() {
}
public Comment(String text) {
this.text = text;
}
//getters and setters
}
Id class
CommentId
public class CommentId implements Serializable {
private static final long serialVersionUID = 1L;
private Post post;
private Long id;
public CommentId(Post post, Long id) {
super();
this.post = post;
this.id = id;
}
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result+ ((post == null) ? 0 : post.hashCode());
result = prime * result ;
return result;
}
public boolean equals(Object object) {
if (object instanceof CommentId) {
CommentId pk = (CommentId)object;
return id.equals(pk.id) && post == pk.post;
} else {
return false;
}
}
//getters and setters
}
repositories
PostRepository
CommentRepository
#Repository
public interface PostRepository extends JpaRepository<Post, Long> {
}
#Repository
public interface CommentRepository extends JpaRepository<Comment, Long>
{
}
Controller class get request and I am using mysql database
#RestController
#RequestMapping("/demo")
public class Controller {
#Autowired
PostRepository ps;
CommentRepository cs;
#GetMapping("/post")
public List<Post> getAll(){
return ps.findAll();
}
#GetMapping("/comment")
public List<Comment> getAllcom(){
return cs.findAll();
}
}