Question

I have a 3 tables as follows

Post
    pid pname 

TagPost
    tagid postId postTime

Tag
    tagid tagNam

One post can have multiple tags and one tag can have multiple posts. Basically a n*n mapping.

Now how can I model following into HQL queries using session.createQuery()

1. Select all challenges which has for a given tag order by creation time of post I can do post.getTagSet(), but I want only n newest posts.
2. Select all post which has java as one of its tags? Order by postTime.

3. Is this right way to model this kind of situation in terms of performance? Or should I have a following structure?

   postid pname postTime tags

Value of tags column in this case can be comma seperated.
Sample value of tags column = java,hibernate

Answer 1

Your fist design with three tables is the better one considering all the situations. You can run queries efficiently. These kinds of queries are very common in HQL.

1. to get n newest posts you can have a query like :

   Query query = session.createQuery("select tagPost.tag from TagPost as tagPost where tagPost.post = :post ORDER by tagPost.postTime");
   query.setMaxResults(n); // n is your expect newest number
   

2. this one is also simple:

   Query query = session.createQuery("select tagPost.post from TagPost as tagPost where tagPost.tag = :tag ORDER by tagPost.postTime");
   query.setMaxResults(n); // n is your expect newest number
   

In both the cases, you need to pass your object parameters to the query before calling query.list(); (first case post, second case tag).

Thanks and happy coding!