Question

I’m just starting out with NHibernate and I’m having trouble with running more complex queries.

I have entities with a list of tags attached. The user will provide two lists of tags, include and exclude.

I need to find all the entities that have all of the include tags, and exclude any entites that have any tag in the exclude list.

Below is my first effort- which is clearly wrong as its listing all Display objects that have any of the include tags rather than all!

Any assistance is greatly appeciated.

var includeTagIds = (from tag in regime.IncludeTags select tag.Id).ToList<int>();
var excludeTagIds = from tag in regime.ExcludeTags select tag.Id;


var displays = session.QueryOver<Display>()
                      .JoinQueryOver<DisplayTag>(display => display.Tags)
                      .WhereRestrictionOn(tag => tag.Id)
                      .IsIn(includeTagIds).List().Distinct();


return displays.ToList();

Answer 1

That query isn’t trivial (have a think about how you might do this using raw SQL). I think the following will work (requiring two correlated sub-queries):

Display displayAlias = null;

var countIncludedTagsSubquery =
    QueryOver.Of<Display>()
        .Where(d => d.Id == displayAlias.Id)
        .JoinQueryOver<DisplayTag>(d => d.Tags)
            .WhereRestrictionOn(t => t.Id).IsInG(includedTagIds)
            .Select(Projections.RowCount());

var excludedTagsSubquery =
    QueryOver.Of<Display>()
        .Where(d => d.Id == displayAlias.Id)
        .JoinQueryOver<DisplayTag>(d => d.Tags)
            .WhereRestrictionOn(t => t.Id).IsInG(excludedTagIds)
            .Select(t => t.Id);

var displays =
    session.QueryOver<Display>(() => displayAlias)
        .WithSubquery.WhereValue(includedTagIds.Count).Eq(countIncludedTagsSubquery)
        .WithSubquery.WhereNotExists(excludedTagsSubquery)
        .List();