NHibernate 3 LINQ inner join issue with three jumps: NotSupportedException

Question

I have a query that used to work in NHibernate LINQ 2.1.2 but it is throwing NotSupportedException with NH3:

    IQueryable<Tree> query = from flower in GetSession().Query<Flower>()
                             from leaf in flower.Stem.Leaves // <--- the problem is here with three jumps
                             where leaf.Color == Green
                             select flower;

The relations are like:

• Flower References Stem
• Stem HasMany Flowers
• Leaf References Stem
• Stem HasMany Leaves

The exception is thrown from line 204 in NHibernate.Linq.Visitors.QueryModelVisitor. Here is the method from the source code:

    public override void VisitAdditionalFromClause(AdditionalFromClause fromClause, QueryModel queryModel, int index)
    {
        if (fromClause is LeftJoinClause)
        {
            // It's a left join
            _hqlTree.AddFromClause(_hqlTree.TreeBuilder.LeftJoin(
                                 HqlGeneratorExpressionTreeVisitor.Visit(fromClause.FromExpression, VisitorParameters).AsExpression(),
                                 _hqlTree.TreeBuilder.Alias(fromClause.ItemName)));
        }
        else if (fromClause.FromExpression is MemberExpression)
        {
            var member = (MemberExpression) fromClause.FromExpression;

            if (member.Expression is QuerySourceReferenceExpression)
            {
                // It's a join
                _hqlTree.AddFromClause(_hqlTree.TreeBuilder.Join(
                                     HqlGeneratorExpressionTreeVisitor.Visit(fromClause.FromExpression, VisitorParameters).AsExpression(),
                                     _hqlTree.TreeBuilder.Alias(fromClause.ItemName)));
            }
            else
            {
                // What's this?
                throw new NotSupportedException(); // <--------- LINE 204
            }
        }
        else
        {
            // TODO - exact same code as in MainFromClause; refactor this out
            _hqlTree.AddFromClause(_hqlTree.TreeBuilder.Range(
                                 HqlGeneratorExpressionTreeVisitor.Visit(fromClause.FromExpression, VisitorParameters),
                                 _hqlTree.TreeBuilder.Alias(fromClause.ItemName)));

        }

        base.VisitAdditionalFromClause(fromClause, queryModel, index);
    }

It seems to me the same issue is discussed under the following thread:

http://groups.google.com/group/nhusers/browse_thread/thread/dbceb7eb1e31f027/f8e69671b750e0d6?lnk=gst&q=NotSupportedException+stefan#f8e69671b750e0d6

Under that thread Stefan mentions that the syntax is not supported:

The LINQ provider expects the expression to be:

QuerySourceReferenceExpression . Member

However, in the case of from brw in loan.Application.Borrowers it is:

QuerySourceReferenceExpression . Member . Member

So it's definately an unsupported feature.

So, is this syntax going to be supported anytime in NH3 LINQ? I think it is a trivial syntax and it's good to have.

However I can go around this issue by rewriting the query as:

        IQueryable<Tree> query = from stem in  GetSession().Query<Stem>()
                                 from leaf in stem.Leaves
                                 from flower in stem.Flowers
                                 where leaf.Color == Green
                                 select flower;

BTW, anyone has a better workaround?

nhusers link: http://groups.google.com/group/nhusers/browse_thread/thread/334a53c749b0b377

Answer 1

After all that effort put in, your question:

is this syntax going to be supported anytime in NH3 LINQ?

... simply cannot be answered in this forum. NHibernate is not a commercial product with a roadmap. You can't just post here and hope one of the volunteer developers responds.

Remember that NHibernate is open source, so the community (including you!) owns issues like this.

I have been following the the nhibernate-development list, and it looks like the LINQ provider is a major area of work. However, I don't know if your specific issue will be addressed. The best way to increase the chances of this issue being fixed is to file a bug in the NHibernate JIRA along with a test case showing the problem.

If it doesn't look like your specific issue will be addressed, why not download the source code and try fixing it yourself, and/or discussing it further on the mailing list? If you download the source code and work with it a little, you'll also find that it has plenty of great example test cases that you can use as examples when you file the bug.