NHibernate get first x distinct results does not get me x results?

Question

I have an ICriteria query like so:

var contentCriteria = DetachedCriteria.For<InvoiceItem>();
var countCriteria = DetachedCriteria.For<InvoiceItem>();

if (model.CurrentPage <= 0) model.CurrentPage = 1;

if (model.OnlyShowErrors)
{
    contentCriteria.Add(Restrictions.Not(Restrictions.Eq("TroubleClass", TroubleClasses.Success)));
    countCriteria.Add(Restrictions.Not(Restrictions.Eq("TroubleClass", TroubleClasses.Success)));
}

if (!string.IsNullOrEmpty(model.BatchId))
{
    contentCriteria.Add(Restrictions.Eq("BatchId", model.BatchId));
    countCriteria.Add(Restrictions.Eq("BatchId", model.BatchId));
}

if (model.DocumentStartDate != null)
{
    contentCriteria.Add(Restrictions.Ge("DocumentDate", model.DocumentStartDate));
    countCriteria.Add(Restrictions.Ge("DocumentDate", model.DocumentStartDate));
}

if (model.DocumentEndDate != null)
{
    contentCriteria.Add(Restrictions.Le("DocumentDate", model.DocumentEndDate));
    countCriteria.Add(Restrictions.Le("DocumentDate", model.DocumentEndDate));
}

if (!string.IsNullOrEmpty(model.VendorId))
{
    contentCriteria.Add(Restrictions.Eq("VendorId", model.VendorId));
    countCriteria.Add(Restrictions.Eq("VendorId", model.VendorId));
}


using (var session = GetSession())
{

    var countC = countCriteria.GetExecutableCriteria(session)
        .SetProjection(Projections.CountDistinct("RecordId"));

    var contentC = contentCriteria
        .AddOrder(Order.Desc("PersistedTimeStamp"))
        .GetExecutableCriteria(session)
        .SetResultTransformer(Transformers.DistinctRootEntity)
        .SetFirstResult((model.CurrentPage * model.ItemsPerPage) - model.ItemsPerPage)
        .SetMaxResults(model.ItemsPerPage);

    var mq = session.CreateMultiCriteria()
        .Add("total", countC)
        .Add<InvoiceItem>("paged", contentC);

    model.Invoices = ((IEnumerable<InvoiceItem>)mq.GetResult("paged"));
    model.Invoices = model.Invoices
        .OrderBy(x => x.PersistedTimeStamp);

    model.TotalItems = (int)(mq.GetResult("total") as System.Collections.ArrayList)[0];
}
return model;

This returns results, but where I would expect the results to be in groups of model.ItemsPerPage, it rarely is. I think that the .SetResultTransformer(Transformers.DistinctRootEntity) transform is being run after the .SetMaxResults(model.ItemsPerPage) limit, and I don't know why or how to fix it. Can someone please enlighten me?

Answer 1

So this is directly related to what was written in this blog post. Additionally, I had the platform-specific complication of PostgreSQL not allowing a DISTINCT ordered set ordered by something not in the SELECT list. Ultimately, I had to make two calls to the database, like so:

using (var session = GetSession())
{
    //I honestly hope I never have to reverse engineer this mess.  Pagination in NHibernate
    //when ordering by an additional column is a nightmare.
    var countC = countCriteria.GetExecutableCriteria(session)
        .SetProjection(Projections.CountDistinct("RecordId"));

    var contentOrdered = contentCriteria
        .SetProjection(Projections.Distinct(
            Projections.ProjectionList()
                .Add(Projections.Id())
                .Add(Projections.Property("PersistedTimeStamp"))
                ))
        .AddOrder(Order.Desc("PersistedTimeStamp"))
        .SetFirstResult((model.CurrentPage * model.ItemsPerPage) - model.ItemsPerPage)
        .SetMaxResults(model.ItemsPerPage);

    var contentIds = contentOrdered.GetExecutableCriteria(session)
        .List().OfType<IEnumerable<object>>()
        .Select(s => (Guid)s.First())
        .ToList();

    var contentC = DetachedCriteria.For<InvoiceItem>()
        .Add(Restrictions.In("RecordId", contentIds))
        .SetResultTransformer(Transformers.DistinctRootEntity);

    var mq = session.CreateMultiCriteria()
        .Add("total", countC)
        .Add("paged", contentC);

    model.Invoices = (mq.GetResult("paged") as System.Collections.ArrayList)
        .OfType<InvoiceItem>()
        .OrderBy(x => x.PersistedTimeStamp);

    model.TotalItems = (int)(mq.GetResult("total") as System.Collections.ArrayList)[0];
}
return model;

This is not pretty, but it worked; I think the folks over at NHibernate need to work on this and make it a tad bit easier.

Answer 2

You need to see the SQL generated by NHibernate as this is not essentially NHibernate bug but behavior of SQL queries when ROWNUM and DISTINCT are applied together. This has been an issue in our known issues list from long.

Following URLs might enlighten you...

ROW NUMBER vs DISTINCT

How ROWNUM works