Fluent nHibernate Query => QueryOver，Join，Distinct

Question

I try to change that query to QueryOver<> to be able to do the Distinct operation yet inside the (generated sql) query

var result = (from x in Session.Query<Events>()
              join o in Session.Query<Receivers>() on x.ID equals o.ID
              where x.Owner.ID == 1 //the user is the owner of that Event (not null)
                    ||
                    x.EVType.ID == 123 //(not null)
                    ||
                    x.Receivers.Count(y => y.User.ID == 1) > 0 //the user is one of the Event Receivers
              select x.StartDate)
              .Distinct();

I tried something like that

Events x = null;
List<Receivers> t = null;

var result = Session.QueryOver<Events>(() => x)
                    .JoinAlias(() => x.Receivers, () => t)
                    .Where(() => x.Owner.ID == 1
                                ||
                                x.EVType.ID == 123
                                ||
                                t.Count(y => y.User.ID == 1) > 0)
                   .TransformUsing(Transformers.DistinctRootEntity)
                   .Select(a => a.StartDate)
                   .List();

but then I got the Value can not be null. Parameter name: source Value can not be null. Parameter name: source exception. Any ideas how can I fix that query ?

edit

thanks to the xanatos' answer, the final SQL query is correct (I used his 2nd approach):

SELECT distinct this_.StartDate as y0_ 
FROM Events this_ 
WHERE 
(
    this_.UserID = ? 
    or
    this_.EventTypeID = ? 
    or
    exists (SELECT this_0_.ID as y0_ 
            FROM Receivers this_0_ 
            WHERE this_0_.UserID = ?)
)

Answer 1

"In QueryOver, aliases are assigned using an empty variable. The variable can be declared anywhere (but should be empty/default at runtime). The compiler can then check the syntax against the variable is used correctly, but at runtime the variable is not evaluated (it's just used as a placeholder for the alias). " http://nhibernate.info/blog/2009/12/17/queryover-in-nh-3-0.html

Setting List<Receivers> t to empty collection as you did (as you have mentioned in comments) means that you check is event id in local empty collection - doesn't have sense at all.

You can try do your query with subquery (should work but i'm not sure, I wrote it without testing, "by hand"):

Receivers receiversSubQueryAlias = null;
var subquery = session.QueryOver<Events>()
                      .JoinQueryOver<Receivers>(x => x.Receivers, () => receiversSubqueryAlias, JoinType.Inner)
                      .Where(()=> receiversSubQueryAlias.UserId == 1)
                      .Select(x => x.Id)
                      .TransformUsing(Transformers.DistinctRootEntity);

Events eventsAlias = null;
var mainQueryResults = session.QueryOver<Events>(() => eventsAilas)
                       .Where(Restrictions.Disjunction()
                             .Add(() => eventAlias.OwnerId == 1)
                             .Add(() => eventAlias.EVType.Id == 123)
                             .Add(Subqueries.WhereProperty<Events>(() => eventAlias.Id).In(subquery))
                        ).Select(x => x.StartDate)
                        .TransformUsing(Transformers.DistinctRootEntity)
                        .List();

Answer 2

Hopefully this answer can help others. This error was being caused by declaring

List<Receivers> t = null;

followed by the query expression

 t.Count(y => y.User.ID == 1) > 0

The QueryOver documentation states "The variable can be declared anywhere (but should be empty/default at runtime)." Since in this case, the place holder is a List, you must initialize it as an empty list.

List<Receivers> t = new List<Receivers>();

Otherwise, when you try to reference the Count method, or any other method on the placeholder object, the source (t) will be null.

This however still leaves a problem as @fex and @xanatos, in which it makes no sense to reference Count() from the alias List t, as it won't convert into SQL. Instead you should be creating a subquery. See their answers for more comprehensive answer.

Answer 3

As written by @fex, you can't simply do a new List<Receivers> . The problem is that you can't mix QueryOver with "LINQ" (the t.Count(...) part). The QueryOver "parser" tries to execute "locally" the t.Count(...) instead of executing it in SQL.

As written by someone else, TransformUsing(Transformers.DistinctRootEntity) is client-side. If you want to do a DISTINCT server-side you have to use Projections.Distinct .

You have to make an explicit subquery. Here there are two variants of the query. the first one is more similar to the LINQ query, the second one doesn't use the Count but uses the Exist (in LINQ you could have done the same by changing the Count(...) > 0 with a Any(...)

Note that when you use a .Select() you normally have to explicitly tell the NHibernate the type of the .List<something>()

Events x = null;
Receivers t = null;

// Similar to LINQ, with COUNT

var subquery2 = QueryOver.Of<Receivers>(() => t)
    .Where(() => t.SOMETHING == x.SOMETHING) // The JOIN clause between Receivers and Events
    .ToRowCountQuery();

var result2 = Session.QueryOver<Events>(() => x)
    .Where(Restrictions.Disjunction()
        .Add(() => x.Owner.ID == 1)
        .Add(() => x.EVType.ID == 123)
        .Add(Subqueries.WhereValue(0).Lt(subquery2))
    )
    .Select(Projections.Distinct(Projections.Property(() => x.StartDate)))
    .List<DateTime>();

// With EXIST

var subquery = QueryOver.Of<Receivers>(() => t)
    .Where(() => t.SOMETHING == x.SOMETHING) // The JOIN clause between Receivers and Events
    .Select(t1 => t1.ID);

var result = Session.QueryOver<Events>(() => x)
    .Where(Restrictions.Disjunction()
        .Add(() => x.Owner.ID == 1)
        .Add(() => x.EVType.ID == 123)
        .Add(Subqueries.WhereExists(subquery))
    )
    .Select(Projections.Distinct(Projections.Property(() => x.StartDate)))
    .List<DateTime>();

Note that you'll have to set "manually" the JOIN condition in the subquery.