Using OneToMany relation in Spring Boot and PostgreSQL

I have two entities, Post and Comment.

Post entity:

@Entity
@Table(name = "posts")
public class Post {

    @Id
    @GeneratedValue
    private Long id;

    private String title;
    private String content;

    @OneToMany(mappedBy = "post", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Comment> comments = new ArrayList<>();

Comment entity:

@Entity
@Table(name = "comments")
public class Comment {

    @Id
    @GeneratedValue
    private Long id;

    private String content;

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

Left out getters and setters for readability.

When I send a POST-request via PostController, I can store a new Post in my PostgreSQL database. How do I add new Comments to this Post via Controller? I seem to not find the answer anywhere.

So, here you have create a two way relationship and hence will need to update both the post entity and comment entity.

let take for instance that your comments path is /post/{postId}/comment, and you are using Sping JPA (with repositories for comment and post as commentRepository and postRepository respectively.)

then controller method will look like -

@PostMapping("/post/{postId}/comment")
public ResponseEntity postController(@PathParam("postId") Long postId,
  @RequestBody Comment comment) {
  Post post = postRepository.getById(postId); 
  comment.setPost(post);
  post.getComments().add(comment);
  commentRepository.save(comment);
  postRepository.save(post);
}

Another alternative is to create a one way relationship, so

@Entity
@Table(name = "posts")
public class Post {

    @Id
    @GeneratedValue
    private Long id;

    private String title;
    private String content;

@Entity
@Table(name = "comments")
public class Comment {

    @Id
    @GeneratedValue
    private Long id;

    private String content;

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

Then, you only need to update the comment entity on POST-request and if you need to get all the comments for a post you can do -

List<Comment> comments = commentRepository.getByPostId(postId);