NHibernate many-to-many criteria

Question

I have a list of questions, each linked to a list of tag.

And the following data :

Question1 : Tag1
Question2 : Tag1, Tag2
Question3 : Tag1, Tag2, Tag3
Question4 : Tag1, Tag3

The following criteria :

var tagsIds = new int[] { tag1, tag2 };

var res = session.CreateCriteria<Question>()
    .CreateCriteria( "Tags" )
    .Add( Restrictions.In( "id", tagsIds ) )
    .List<Question>();

returns (I understand why, the "in" acts like an OR)

Question1, Question2, Question3, Question4

Or I would like to get only

Question2, Question3

as they both have tag1 AND tag2. I there a way to do it ?

In SQL, I would do something like :

SELECT *
FROM Question q
WHERE EXISTS (
    SELECT *
    FROM QuestionsToTags qtt
    WHERE qtt.Question_id = q.Id
    AND qtt.Tag_id IN ( 1, 2 )
    GROUP BY qtt.Question_id
    HAVING COUNT( qtt.Question_id ) >= 2 
)

score 1 · Answer 1 · answered Dec 10 '10 at 13:10

If you only have two then use an And restriction

var res = session.CreateCriteria<Question>()
    .CreateCriteria( "Tags" )
    .Add( Restrictions.And(Restrictions.Eq("id", tag1), Restrictions.Eq("id", tag2))
    .List<Question>();

If you have more than two or an unknown number then use Conjunction:

var conj = new Conjunction();
// use a loop to add all tags if number is unknown
conj.Add(Restrictions.Eq("id", tag1);
conj.Add(Restrictions.Eq("id", tag2);

var res = session.CreateCriteria<Question>()
    .CreateCriteria( "Tags" )
    .Add(conj)
    .List<Question>();

There is also a Disjunction class to handle multiple Or conditions.

Thank you, but this isn't working, as it generates the following SQL : SELECT [snipped] FROM "Question" this_ inner join QuestionsToTags tags3_ on this_.Id=tags3_.Question_id inner join "Tag" tag1_ on tags3_.Tag_id=tag1_.Id WHERE (tag1_.Id = 1 and tag1_.Id = 2). So nothing is returned, as id can't be both 1 and 2 — mathieu, Dec 11 '10 at 19:05

score 0 · Accepted Answer · answered Dec 13 '10 at 11:22

Using hql :

var q = NHibernateSession.CreateQuery(
@"from Question question 
    where exists( 
        select q.id from Question q
        join q.Tags t
        where 
            t.id in (:ids)
            and q.id = question.id
        group by q.id
        having count(t.id)=:c_count )");

q.SetParameterList("ids", tagIds);
q.SetInt32("c_count", tagIds.Length);

And using an ICriteria :

// here is the exists part
var dCriteria = DetachedCriteria.For<Question>("q")
    .SetProjection(Projections.GroupProperty(Projections.Id()))
    .Add(Restrictions.Eq(Projections.Count(Projections.Id()), tagIds.Length))
    // here we filter on the "parent" criteria
    .Add(Restrictions.EqProperty("q.id", "question.Id"))
    .CreateCriteria("Tags")
    .Add(Restrictions.In("id", tagIds));

var crit = NHibernateSession
    .CreateCriteria<Question>("question")
    .Add(Subqueries.Exists(dCriteria));

NHibernate many-to-many criteria

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