Is it possible to use NH QueryOver to fetch joined entities in one query? - sql

Question

SQL query is:
select B.* from A inner join B on A.b_id = B.id where A.x in (1,2,3)
A <-> B relation is many-to-one
I need to filter by A but fetch related B.
UPDATE:
I tried this NH QueryOver
Session.QueryOver<A>.Where(a => a.x.IsIn(array)).JoinQueryOver(a => a.B).Select(a => a.B).List<B>()
but it results in a N+1 sequence of queries: the first one fetches IDs of related Bs, and others fetch related Bs one by one by ID (analyzed via NHProf). I want it to fetch a list of Bs in one go.
UPDATE 2:
for now I worked around this with subquery
Session.QueryOver(() => b).WithSubquery.WhereExists(QueryOver.Of<A>().Where(a => a.x.IsIn(array)).And(a => a.b_id == b.id).Select(a => a.id)).List<B>()
but I still hope to see an example of QueryOver without subquery as I tend to think subquery is less efficient.

Answer 1

This works (at least in my test application):
var list = session.QueryOver<A>()
.Where(a => a.X.IsIn(array))
.Fetch(x => x.B).Eager
.List<A>()
.Select(x => x.B);
Note that the .Select() statement is a normal LINQ statement, not part of NHibernate.
Generated SQL:
SELECT
this_.Id as Id0_1_,
this_.B as B0_1_,
this_.X as X0_1_,
b2_.Id as Id1_0_,
b2_.SomeValue as SomeValue1_0_
FROM A this_ left outer join B b2_ on this_.B=b2_.Id
WHERE this_.X in (?, ?, ?)
It's not optimal if A is a very large class, of course.
An NHibernate-only solution with a subquery:
var candidates = QueryOver.Of<A>()
.Where(a => a.X.IsIn(array))
.Select(x => x.B.Id);
var list = session.QueryOver<B>()
.WithSubquery.WhereProperty(x => x.Id).In(candidates).List();
I'll try to find the reason why the most obvious solution (just adding Fetch().Eager) doesn't work as expected. Stay tuned!