I’m replying to a post in the DevExpress XPO forum, where the question came up of how to use joins with XPO/LINQ and projection. Unfortunately, the forum system mangled my code snippets, so I thought I’d put the info up here as well.

Emilio Garcia wrote:

I trying to create a linq query to retrive some objects, this is the query:

var
listadoSesiones = from u in usuariosjoin s in sesiones on u.ObjId equals
s.UsuarioIdwhere s.Activa == true
orderby s.FechaInicio ascending
select new { u.NombreUsuario, u.NombreCompleto, s.FechaInicio, s.FechaFin };

I’m pretty sure this doesn’t have anything to do with the Guid type.
Instead, the problem is the projection you’re trying to do. Instead, you
should define your classes correctly and things should be much easier…

I’m roughly translating your names, and I’m coming up with classes that
look like this:

Code:

Note the two properties marked with [Association] - these are the ones
that implement the relationship for XPO. You should always set these
relationships up correctly, then it will be much easier to get to the
right data.

(As an aside - when working with legacy databases, you’ll sometimes need
to use the [Persistent(...)] attribute on properties like this to define
the name of the foreign key fields in your database.)

Once you’ve got your classes set up like this, you can easily query the
information you were looking for, utilizing these pre-configured
relationships:

Code:

Of course on the other hand there’s no real need to do this either -
it’s so easy to get to the data you need through the relationship
properties, you don’t need to work with projection.