I want to find the list of Child using parent’s id using spring data jpa.
I know there are two ways to retrieve data from spring data jpa, which is
using

1. derived queryies
2. query methods like findBy….()
   or using a native query @Query(“[]”).

These are the two entities I need. Post, and Reply Entitiy.

Post is ManyToOne to Reply, and Reply is OneToMany To Post.
I also have a Member entity in oneToManyRelation from Post Entity which is not used in this case though.

Post

@Entity
@NoArgsConstructor
@AllArgsConstructor
@Builder
@Getter
@Setter
@ToString(exclude = {
  "member"}
) public class Post extends BaseEntity {
  @Id
  @GeneratedValue
  private Long postId;
  @ManyToOne(fetch = FetchType.LAZY) @JoinColumn(name = "member_id") private Member member;
  @Column(nullable = false, columnDefinition = "TINYTEXT") private String title;
  @Column(nullable = false, columnDefinition = "TEXT") private String content;
  @Column(nullable = false) private Long views = 0L;
  @OneToMany(mappedBy = "post") private List<Reply> replies;
}

Reply

@Entity
@NoArgsConstructor @AllArgsConstructor @Builder
@Getter
@Setter
@ToString(exclude = {"member"})
public class Reply extends BaseEntity {
    @Id @GeneratedValue
    private Long replyId;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "member_id")
    private Member member;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "post_id")
    private Post post;
    @Column(length = 500)
    private String replyText;
}

I want to query like code below work. However, this native query shows error.

    @Query("SELECT p FROM Reply r JOIN Post p WHERE r.post.postId= :postid")
    List<Reply> findRepliesByPostId(@Param("postId") Long postId, Sort sort);

Error

Could not create query for public abstract java.util.List com.cms.mini_board.repository.ReplyRepository.findRepliesByPostId(java.lang.Long,org.springframework.data.domain.Sort); Reason: Using named parameters for method public abstract java.util.List com.cms.mini_board.repository.ReplyRepository.findRepliesByPostId(java.lang.Long,org.springframework.data.domain.Sort) but parameter 'Optional[postId]' not found in annotated query 'SELECT p FROM Reply r JOIN Post p WHERE r.post.postId= :postid'

I have succeeded in returning what I wanted List<Reply> by operating in Post Entity in postRepository.
However, I want the same result operating in Reply Repository because I think it suits the meaning of “searching from Reply Table where postId = postId”. Which is the case when using RDBMS. Below is the code I succeeded using Post Entity.

    @Override
    public List<ReplyDTO> getReplyList(Long postId) {
        Post postResult = postRepository.findById(postId)
                .orElseThrow(() -> new ResponseStatusException(HttpStatus.NO_CONTENT, "No such element exception"));
        List<ReplyDTO> replyDTOS = postResult.getReplies().stream().map(reply ->
            replyEntityToDTO(reply)
        ).collect(Collectors.toList());
        return replyDTOS;
    }

So the final question is,

1. Can I do this in query methods / derived query methods?
2. Or Is there any method to do this?
3. I can’t find why my code below doesn’t work. can someone tell the solution?

@Query("SELECT p FROM Reply r JOIN Post p WHERE r.post.postId= :postid")
    List<Reply> findRepliesByPostId(@Param("postId") Long postId, Sort sort);

4. Is searching for Child entity list by Parent’s Id is bad practice because I am thinking in a RDBMS way, and not thinking in Hibernate/JPA way?