Pulling a property up to the parent object level - nhibernate

Question

Take the following NHibernate scenario:
I have two tables:
Header
HeaderID (int),
ForeignKey1 (int),
ForeignKey2 (int)
Child
ForeignKey1 (int),
ForeignKey2 (int)
Description (varchar)
...
Metric Shed Ton Of Other Columns That Im Not InterestedIn(various types)
I'm using Fluent NHibernate - I'd like to project the value of Description on the Child object into the parent (is that the correct terminology?!) - but obviously Child contains a lot of data in its columns - I don't want all that extra data, just the description...how do I get NH to produce the equivalent of the following query:
select
Header.*, Child.Description
from
Header
inner join
Child ON Header.ForeignKey1 = Child.ForeignKey1 AND Header.ForeignKey2 = Child.ForeignKey2
The only way I've got this working up to now is to use a Reference mapping to reference the child from the Header entity and then just created a non-mapped property on Header which pointed to Child.Description. Obviously this means that NH fetches the whole child object before it can query the value of Description
(I don't think the composite key is a problem here, the join seems to work fine - it's just how to get the data without getting all the non-interesting data)
At the moment my header entity looks like this:
public virtual int HeaderID { get; set; }
public virtual int KeyColumn1 { get; set; }
public virtual int KeyColumn2 { get; set; }
public virtual Child Child { get; set; }
public virtual string Description { get { return Child.Description; } }
The mappings for it:
Id(x => x.HeaderID);
Map(x => x.KeyColumn1);
Map(x => x.KeyColumn2);
References<Child>(x => x.Child).Fetch.Join().Columns("KeyColumn1", "KeyColumn2").ReadOnly();
Basically, I can't change the schema, I'm not interested in the rest of the data in the Child table, and I can't create views (can't change schema)
Anyone have any ideas if this is possible? I'm going to be querying a big list of Header objects, and I need that field from Child but I don't want the query to take forever!
Is this something I would have to do at the query level instead using hql or crit API?
Edit:
Trying to get this working using query API
Header.Session.QueryOver<Header>(() => parent)
.Where(h => h.HeaderID == 1)
.Inner.JoinQueryOver(x => x.Child, () => child)
.Select(x => x.HeaderID, x => x.ForeignKey1, x => x.ForeignKey2, x => child.Description);
Checking the SQL shows the query is exactly what I want - but I get an exception System.Object[] is not of type Header and cannot be used in this generic collection
I assume this is because what I'm getting is just an array of values from the Select() - any idea how I can transform this into a Header object?
Edit: I ended up with
Header.Session.QueryOver<Header>(() => parent)
.Where(h => h.HeaderID == 1)
.Inner.JoinQueryOver(x => x.Child, () => child)
.Select(x => x.HeaderID, x => x.ForeignKey1, x => x.ForeignKey2, x => child.Description)
.TransformUsing(new GenericResultTransformer(typeof(Header), "HeaderID", "ForeignKey1", "ForeignKey2", "Description"));
Which works just how I want it - if anyone has a better suggestion I'm open to it, but like I said, I can't touch the schema at all

Answer 1

There is the concept of lazy properties, but that is really for the opposite situation, i.e. there is a small number of properties that you want to exclude.
If this is for a presentation scenario, use can use any of NHibernate's query methods, to project just the columns you like - the query result need not be a complete entity. See for instance the select clause in HQL and LINQ, and the SetProjection() family in Criteria/QueryOver.