Question

The data structure is as follows:
A house has many rooms. Each room has many persons.

What I want to do is to get all persons for a house. In plain SQL I would write the following:

SELECT * FROM Person WHERE Room_id
IN
(SELECT Id FROM Room WHERE House_id = 1)

How can I write that in Fluent NHibernate’ish code?

For this example, we can assume that the entities and mappings look like this:

House entity

public virtual int Id { get; set; }
public virtual string Name { get; set; }
public virtual IEnumerable<Room> Rooms { get; set; }

House mapping

Id(x => x.Id);
Map(x => x.Name);
HasMany(x => x.Rooms);

Room entity

public virtual int Id { get; set; }
public virtual string Name { get; set; }
public virtual House House { get; set; }
public virtual IEnumerable<Person> Persons { get; set; }

Room mapping

Id(x => x.Id);
Map(x => x.Name);
References(x => x.House);
HasMany(x => x.Persons);

Person entity

public virtual int Id { get; set; }
public virtual string Name { get; set; }
public virtual Room Room { get; set; }

Person mapping

Id(x => x.Id);
Map(x => x.Name);
References(x => x.Room);

Answer 1

To get SQL query close to yours you can use these criterias:

var subCriteria = DetachedCriteria.For<Room>(); // subquery
subCriteria.Add(Expression.Eq("House", house)); // where clause in subquery
subCriteria.SetProjection(Projections.Id()); // DetachedCriteria needs to have a projection, id of Room is projected here

var criteria = session.CreateCriteria<Person>();
criteria.Add(Subqueries.PropertyIn("Room", subCriteria)); // in operator to search in detached criteria
var result = criteria.List<Person>();

This produces something like this:

SELECT this_.Id as Id4_0_, this_.Name as Name4_0_, this_.RoomId as RoomId4_0_
FROM [Person] this_
WHERE this_.RoomId in (SELECT this_0_.Id as y0_ FROM [Room] this_0_ WHERE this_0_.HouseId = @p0)',N'@p0 int',@p0=1

I tested it in FNH1.2 and NH3.1 but it should work well in NH2.1 as well.

EDIT:
UpTheCreek is right. Linq is more clear than Criteria API. For example:

var query = session.Query<Person>().Where(x => x.Room.House == house);
var linqResult = query.ToList<Person>();

which produces different SQL query but result set is the same:

select person0_.Id as Id4_, person0_.Name as Name4_, person0_.Room_id as Room3_4_
from [Person] person0_, [Room] room1_
where person0_.Room_id=room1_.Id and room1_.House_id=2