Many to many relation, multi where clause on the same column and hibernate - sql

Question

Sorry for the bad question title, couldnt think of anything better.
Anyway, my tables are Tags - Poststags - Posts. Poststags is a junction table for many to many relation. I need to select all posts with given tags, I dont know how many tags the user will choose to search for. One way I found to do this is in the code below, however i would need to loop all the tags given by the user and construct the query string from there since the number of tags is unknown. Seems like a pretty bad solution to me.
Another solution would be to store all tags in one column in the Posts table as a pure string, but I dont want to do that because of other application requirements.
I have a working sql query, since I was trying pure sql before trying to implement it in hibernate, but I dont like doing a select of all posts containing each tag and then joining each query, is there a way to specify the same column multiple times in the WHERE clause? Something along the lines WHERE pt.tag_id = x AND pt.tag_id = y? (I know this won't work). IN operator won't work either since it will give me Posts that contain any of the supplied tags and not just the posts containing ALL of the supplied tags.
Also how would I implement such a query in HQL(if subqueries like this are even supported?). Or can I somehow manage this via criteria? Or do I have to resort to using createSQLQuery method of a hibernate session?
SELECT * FROM
( SELECT * FROM posts p
inner join poststags pt on pt.post_id = p.id
WHERE pt.tag_id = 1 ) AS A
INNER JOIN
( SELECT * FROM posts p
inner join poststags pt on pt.post_id = p.id
WHERE pt.tag_id = 2 ) AS B ON A.id = B.id
And yes, I know this query is not returning the Post entity itself, but I can handle that later.

Answer 1

Don't use hibernate or ORM for this kind of complex select, it may work, but in a bad way.
Your use case should be solved by full text search, which means each Post will need have its own tags.
I don't see much value to make Tag an entity. It's just a string.
Full text search could be heavy for database , A better way is using elasticsearch to help. Spring has integration with spring-data-elasticsearch and it's not difficult to use. Elasyicsearch is very powerful for free text search.

Answer 2

Here is a solution that 'should' work using Criteria queries in Hibernate.
Assuming that you have an entity for Post and an entity for PostTag and PostTag has reference to Post (which I think it should given the example query that you provided), I believe that something like this should do what you want:
static DetachedCriteria getPostTagCriteria(String tagString)
{
DetachedCriteria criteria = DetachedCriteria.forClass(PostTag.class, "uniqueName_" + postTagId);
criteria.createAlias("tag", "tag");
criteria.add(Restrictions.eq("tag.tagString", tagString));
criteria.setProjection(Projections.property("postId"));
return criteria;
}
static List<Post> getPosts(List<String> tagStrings)
{
Criteria criteria = getCurrentSession().createCriteria(Post.class, "post");
for(String tagString : tagStrings)
{
criteria.add(Property.forName("post.id").in(getPostTagCriteria(tagString)));
}
List<Post> ret = criteria.list();
return ret;
}
This assumes that you have reasonable entities to represent Post, PostTag and Tag and that they all reference each other in obvious parent/child sort of ways that I have completely made up here.
But, the general idea of creating multiple detached criteria objects based on your input should solve your problem. This solution also comes with the same caveats regarding SQL complexity mentioned above. You will be creating a sub-query for each tag passed in. So, depending on your indexes and table sizes, you may need to consider a different approach.