ERROR: syntax error at or near "." - JPA Pageable - spring

Question

repository:
#Repository
public interface PostRepository extends PagingAndSortingRepository<Post, Long> {
#Query(value = "SELECT p.postComments FROM Post p WHERE p.webId = ?1")
Page<PostComment> findCommentsByWebId(String webid, Pageable pageable);
}
Post entity:
public class Post {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "id")
private Long id;
#Column(name = "web_id")
private String webId;
#Column(nullable = false, name = "title")
private String title;
#Column(nullable = false, name = "description")
private String description;
#Column(nullable = false, name = "mature")
private boolean mature;
#OneToOne(mappedBy = "post")
private Cover cover;
#ManyToOne
#JoinColumn(name = "user_id")
private User user;
#OneToMany(mappedBy = "post")
private List<PostView> postViews;
#ManyToMany
#JoinTable(name = "post_tag",
joinColumns = #JoinColumn(name = "post_id"),
inverseJoinColumns = #JoinColumn(name = "tag_id"))
private List<Tag> tags;
#OneToMany(mappedBy = "post")
private List<PostDownvote> postDownvotes;
#OneToMany(mappedBy = "post")
private List<PostUpvote> postUpvotes;
#OneToMany(mappedBy = "post")
private List<PostComment> postComments;
#Column(name = "created_at")
private Timestamp createdAt;
#Column(name = "updated_at")
private Timestamp updatedAt;
}
The problem: When returning plain List<PostComment> from the query method everything works fine. But if I change it to Page<PostComment> (I need total elements count), I get the following error:
2022-08-03 22:29:41.399 ERROR 9192 --- [nio-8080-exec-3] o.h.engine.jdbc.spi.SqlExceptionHelper : ERROR: syntax error at or near "."
Position: 14
Hibernate: select tags0_.post_id as post_id1_6_0_, tags0_.tag_id as tag_id2_6_0_, tag1_.id as id1_10_1_, tag1_.name as name2_10_1_ from post_tag tags0_ inner join tag tag1_ on tags0_.tag_id=tag1_.id where tags0_.post_id=?
org.springframework.dao.InvalidDataAccessResourceUsageException: could not extract ResultSet; SQL [n/a]; nested exception is org.hibernate.exception.SQLGrammarException: could not extract ResultSet
It is very difficult to debug this. Does anyone have any clue on what is wrong?
I need BOTH paging and total amount of elements.

Answer 1

Basically you are not able to fetch the part of the inner collection. But you could reach it from the another side of the bi-directional relationship
#Repository
public interface PostCommentRepository extends PagingAndSortingRepository<PostComment, Long> {
#Query(value = "SELECT pc FROM PostComment pc WHERE pc.post.webId = ?1")
Page<PostComment> findCommentsByWebId(String webid, Pageable pageable);
// or better using Spring Data naming conventions just
Page<PostComment> findAllByPostWebId(String webid, Pageable pageable);
}

Answer 2

If you only need a total count you should avoid querying list of entities which could be very memory intensive.
So in your PostCommentRepository try the following:
long countAllByPost_WebId(String webId);