NHibernate QueryOver Subquery

Question

I\'ve looked at the similar questions, but can\'t find a simple explanation. I could have missed it, but I promise I looked. Actually I can\'t even find the documentation ot

Answer 1

Well, hashed at this some more, and while I don't like one part of the results, it does work:

var distinctProgIdsSubQuery = QueryOver.Of().
JoinQueryOver(p => p.Topics).
WhereRestrictionOn(pt => pt.Id).IsIn(topicIds)
.Select(Projections.Distinct(Projections.Property(p => p.Id)));


ProgramDTO pDTO = null;
var progQuery = Session.QueryOver()
    .WithSubquery.WhereProperty(p => p.Id).In(distinctProgIdsSubQuery)
    .SelectList(list => list
        .Select(program => program.Id).WithAlias(() => pDTO.Id)
        .Select(...)
        )
    .TransformUsing(Transformers.AliasToBean(typeof(ProgramDTO)));


return progQuery.List();

This produces

SELECT this_.ProgramId as y0_, ...
FROM Programs this_ 
WHERE this_.ProgramId in (
        SELECT distinct this_0_.ProgramId as y0_ 
        FROM
            Programs this_0_ 
        inner join
            Programs_Topics topics3_ 
                on this_0_.ProgramId=topics3_.ProgramId 
        inner join
            Topics topic1_ 
                on topics3_.TopicId=topic1_.TopicId 
        WHERE
            topic1_.TopicId in (
                @p1, @p2, ...
            )
    ) 

This may be a limitation of NH, but there's no need to join the Programs table in the subquery. I tried to write this from the other direction -- that is, to create a QueryOver.Of(), but I could not figure out how to select the program IDs at the end -- select was only giving me the TopicIds, and even then the query was still joining all three tables.

I'm not sure if MS-SQL's query optimizer will avoid the useless join or not, but it would be nice if we didn't have to rely on it.

For now though, this works, and hopefully someone else has fewer headaches than I did trying to figure this out.