Where-clause for LINQ-to-NHibernate FetchMany? - linq

Question

Using LINQ-to-NHibernate is there a way to narrow down what FetchMany() returns?
Given the following class structure
public class Foo
{
public virtual int Id { get; set; }
public virtual IList<Bar> Bars { get; set; }
}
public class Bar
{
public virtual string Description { get; set; }
}
How can I do this:
session.Query<Foo>()
.Where(foo => foo.Id > 30)
.FetchMany(foo =>
foo.Bars.Where(bar => bar.Description.StartsWith("x")));
And NHibernate will return all Foo's with an Id > 30 and for those Foo's all the attached Bar's where the Bar's description starts with the letter 'x'?
I found some posts that use the old QueryOver() stuff but I explicitely want to use NHibernate's LINQ provider.
Any ideas?
Update
I think I need to clarify what I want as a result.
<Foo Id="1">
<Bar Description="x1"/>
<Bar Description="b1"/>
</Foo>
<Foo Id="31">
<Bar Description="x2"/>
<Bar Description="x3"/>
<Bar Description="b2"/>
</Foo>
<Foo Id="32">
<Bar Description="b3"/>
</Foo>
From the data outlined above I expect the following result
<Foo Id="31">
<Bar Description="x2"/>
<Bar Description="x3"/>
</Foo>
<Foo Id="32"/>
The additional Where clause should only work on the Bar's! It should not further narrow down the list of Foo's! Just reduce what FetchMany() returns.

Answer 1

I'm fairly sure you're out of luck with the current linq provider - but for a non-linq (and cross-cutting) option, you might want to have a look at the filter functionality included in NHibernate - it would probably be the best bet for implementing this in a large scale / complex project.

Answer 2

var query = from foo in session.Query<Foo>()
where foo.Id >30
from bar in foo.Bars
where bar.Description.StartsWith("x")
select new { Id = foo, Bar = bar};
var results = query.ToList().ToLookup(x => x, x => x.Bar);
foreach(var foo in results.Keys)
{
var barsWhichStartWithX = results[foo];
//DO STUFF
}
Although this may produce inefficient SQL (I don't use nHibernate so I wouldn't know). You could also try this...Also the above would miss out foos without bars.
var foosQuery = session.Query<Foo>().Where(foo => foo.Id > 30);
var foos = foosQuery.Future();
var barsQuery = from f in foosQuery
from bar in foo.Bars
select new { Id = foo.Id, Bar = bar};
var foosDict = foos.ToDictionary(x => x.Id);
var bars = barsQuery.ToList().ToLookup(x => foosDict[x.Id], x => x.Bar);
foreach(var foo in foos)
{
var barsWhichStartWithX = bars[foo];
//DO STUFF
}

Answer 3

Perhaps not exactly what you're after, but certainly worth considering is the NHibernate.CollectionQuery library.
It allows you to query uninitialized NHibernate collections on an entity using Linq - but would require an additional query/round-trip to get them (unlike FetchMany, which grabs the entire collection in the same round-trip).

Answer 4

You will need a reference from child object to the parent.
var result = from foo in session.Query<Foo>().FetchMany(f => f.Bars)
from bar in session.Query<Bar>()
where foo.Id == bar.FooId && // FooId is missing in your entity
foo.Id > 30
bar.Description.StartsWith("x")
select foo;