QueryOver: select columns from subquery

Question

How can I select / project values from a subquery from a different table into my main query?

I have an NH-model like this:

[Serializable]
public clas         


        

Answer 1

One way would be to use Projections, Subquery and DTO. So let's say, that we have DTO (almost the same as MyModel, but with new extern property ... e.g. Count). Then we can do it like this:

MyModel main = null;
MyModelDTO dto = null;

// the main query
var query = session.QueryOver(() => main);

// the subquery used for projection
var subquery = QueryOver.Of()
    // select something, e.g. count of the ID
    .SelectList(selectGroup => selectGroup.SelectCount(o => o.ID))
    // some condition
    // kind of JOIN inside of the subquery
    .Where(o => o.xxx == main.yyy); // just example

// now select the properties from main MyModel and one from the subquery
query.SelectList(sl => sl
      .SelectSubQuery(subquery)
         .WithAlias(() => dto.Count)
      .Select(() => main.ID)
        .WithAlias(() => dto .ID)
      ....
    );

// we have to use transformer
query.TransformUsing(Transformers.AliasToBean())

// and we can get a list of DTO
var list = query.List();