NHibernate 3. Alternatives to "ThenFetch" in QueryOver

Question

I'm using NHibernate 3.0 with both the LINQ provider and QueryOver. Sometimes I want to eager load related data, and there comes the method "Fetch" to the rescue, both in LINQ and QueryOver. Now I have a special scenario where I want to eager load a property not directly on the second level, like:

Foo f = ...;
f.A.B.C

with LINQ there's no problem, as you can "chain" fetches with the method "ThenFetch", like:

var result = Session.Query<Foo>().Fetch(a => a.A).ThenFetch(b => b.B).ThenFetch(c => c.C).ToList();

In QueryOver there's no such method, so how can I achieve the same result?

Thanks in advance.

Cameral

Answer 1

I actually managed to solve this problem using two different approaches:

Approach one:

Session.QueryOver<Foo>().Fetch(x => x.A).Fetch(x => x.A.B).Fetch(x => x.A.B.C)

Approach two:

A a = null;
B b = null;
C c = null;

Session.QueryOver<Foo>()
    .JoinAlias(x => x.A, () => a)
    .JoinAlias(() => a.B, () => b)
    .JoinAlias(() => b.C, () => c)

Both work (altough I'm not exactly sure if one of them generated "inner" and the other one "outer" joins).

Cameral

Approach two seems to only work for One to One, it fails on collections. – Mohawk

I take that back, it does work if you specify the join type for the collection, by default it attempts an inner join. If you specify a left outer join it works perfectly. – Mohawk

Isn't the FetchType (Eager, Lazy, Default) needed after Fetch()? – Inhale

Nope. At least it wasn't when I posted this answer (with NH 3.0) – Cameral

While this answer is correct, in my own case I was specifically looking to fetch the child of a collection or list. For this case, @xanatos points out that you simply need to access the first element of the collection/list in order to make the C# compiler happy: query .Fetch(x => x.SomeEnumerable) .Fetch(x => x.SomeEnumerable.First().SomeProperty) When NHibernate actually examines this lambda expression, it ignores the call to .First() and simply looks at the property chain. Realizing this made a huge difference for me. – Gownsman

Answer 2

Just as a curiosity, I'll post the reply they gave me on the NHibernate Jira:

query 
    .Fetch(p => p.B) 
    .Fetch(p => p.B.C) // if B is not a collection ... or 
    .Fetch(p => p.B[0].C) // if B is a collection ... or 
    .Fetch(p => p.B.First().C) // if B is an IEnumerable (using .First() extension method) 

Spasmodic

Answer 3

I think you can do that with JoinQueryOver

IQueryOver<Relation> actual =
   CreateTestQueryOver<Relation>()
    .Inner.JoinQueryOver(r => r.Related1)
    .Left.JoinQueryOver(r => r.Related2)
    .Right.JoinQueryOver(r => r.Related3)
    .Full.JoinQueryOver(r => r.Related4)
    .JoinQueryOver(r => r.Collection1, () => collection1Alias)
    .Left.JoinQueryOver(r => r.Collection2, () => collection2Alias)
    .Right.JoinQueryOver(r => r.Collection3)
    .Full.JoinQueryOver(r => r.People, () => personAlias); 

Cence