Count of many to many references

Question

While creating my first spring boot application, I stumbled upon problem of counting many to many references in Tag entity.

@Entity
public class Tag {
    @Id
    @GeneratedValue
    private long id;
    private String name;
    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(name = "tag_post", joinColumns = @JoinColumn(name = "tag_id"), inverseJoinColumns = @JoinColumn(name = "post_id"))
    private List<Post> posts;
    private long weight;

in spring data repository i would like to set weight equal to number of references to given Tag

@Repository
public interface TagRepository extends JpaRepository<Tag, Long> {
    @Modifying
    @Query("Update Tag t SET t.weight = ( SELECT COUNT() FROM Post p WHERE t IN p.tags )")
    // i want to make above query work
    void updateWeights();

I could simply iterate through all tags using findAll() and and set weight ony my own, but this would not be probably optimal solution.

What should I do in order to make this query work? I appreciate any advices.

(i hope my question is not gonna be deleted due to being too trivial)

Answer 1

As workarround to my problem (which was JPQL incompetence actually) i decided to just create method like this.

@Override
public void updateWeights()
{

    for(Tag tag: tagRepository.findAll())
    {

        tag.setWeight(0);
        tagRepository.save(tag);
    }

    List<Post> posts = postRepository.findAll();

    for(Post post:posts)
    {
        List<Tag> tags = post.getTags();
        for(Tag tag: tags)
        {
            tag.setWeight(tag.getWeight()+1);
            tagRepository.save(tag);
        }

    }

}