How to query an M:N relationship with JPA2?

Question

I have an an object (BlogPost) that contains an M:N collection of elements (Tags).

How to query for an object (BlogPost) where at least one it its Tags matches an element in a set of Tags (defined by the user) with JPA2 (Hibernate).

findBlogPostWithAtLeastOneMatchingTag(Collection<Tag> tags){ ???? }

My main problem is, that I actually need to compare two collections of tags: - the collection of tags of the BlogPost. - the collection I search for

I tried Select p from Post p where p.tags in(:tags) but it does not work, as my post entities have more than just one tag.

So what could I do instead?

My BlogPost entity looks like this. It has several Tags.

@Entity
public class BlogPost{

    /** The tags. */
    @ManyToMany()
    @NotNull
    private Set<Tag> tags;

    @NotBlank
    private String content;

    ...
}

---

The solution must not be JPQL, JPA-Criteria (not Hibernate-Criteria) would be fine too.

Answer 1

If you like JPA Criteria, this is the solution for you:

List<Integer> myTagsIds = new ArrayList<Integer> ();
myTagsIds.add(1);
myTagsIds.add(2);

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<BlogPost> cq = cb.createQuery(BlogPost.class);
Root<BlogPost> blogPost = cq.from(BlogPost.class);
SetJoin<BlogPost, Tag> tags = blogPost.join(BlogPost_.tags);
Predicate predicate = tags.get(Tag_.id).in(myTagsIds);
cq.distinct(true);
cq.where(predicate);
TypedQuery<BlogPost> tq = em.createQuery(cq);
return tq.getResultList();

This solution makes use of the canonical MetaModel classes BlogPost_ and Tag_ that should be generated by your JPA implementation.

Answer 2

Approach 1:

In SQL it could be something like:

SELECT p FROM Post p WHERE (p.tags INTERSECT :tags IS NOT EMPTY);

Then apply the @SqlResultSetMapping.

Approach 2:

You can use Criteria API and start as you did but make a loop over Collection<Tag> tags as:

* make a union of single query results from `Select p from Post p where p.tags in(:tags)`;
* take distinct over result of union.

Query will be server-side and you wouldn't have to do dirty work in Java.

Answer 3

You could do something like

Select t from Post t where t.tag in (select p.tag from Post p where p.id=:id)

id is the id of the current post. Basically you are selecting post with tags that are in the tags of current post.