JsonResult from lambda expression using join - asp.net

Question

I am using the following Lambda expression to try and join two models (LyncUser) and (DDI) and only return the DDI numbers that exist in both models. In LyncUser the field is called LyncUser.DDI and in DDI it is called DDI.Number.
This works and returns a list of all DDI numbers that are used from LyncUsers controller.
var lyncDB = new LyncUserEntities();
return Json(lyncDB.lyncUsers
.Select(c => new { DDI = c.DDI }), JsonRequestBehavior.AllowGet);
When I change this to incorporate a join so only the DDI numbers are returned that exist in LyncUsers but do not exist in DDI it fails to bring anything back.
public JsonResult GetAvailableDDINumbers()
{
var lyncDB = new LyncUserEntities();
return Json(lyncDB.LyncUsers
.Join(lyncDB.DDIs, avail => avail.DDI, used => used.Number,
(used, avail) => new { used = used, avail = avail })
.Where(joined => joined.used.DDI == joined.avail.Number), JsonRequestBehavior.AllowGet);
}

Answer 1

only the DDI numbers (...) that exist in LyncUsers but do not exist in DDI
To me that means you want to return LyncUsers having a DDI that doesn't exist as DDI.Number.
Your code, an inner join, does exactly the opposite. It returns the entities where both values (DDI and Number) are equal. And apparently, at this moment, there aren't any meeting this condition.
To achieve what you want you should use a different statement:
return Json(lyncDB.LyncUsers
.Where(u => !lyncDB.DDIs.Any(ddi => ddi.Number == u.DDI)), JsonRequestBehavior.AllowGet);
This should currently return all lyncDB.LyncUsers.

Answer 2

I don't think you need Join at all as you are not using joined result.
Try this instead:
var lyncDB = new LyncUserEntities();
var result = lyncDB.lyncUsers
.Where(x => !lyncDB.DDIs.Any(y => y.Number == x.DDI))
.Select(x => new { DDI = x.DDI });
return Json(result, JsonRequestBehavior.AllowGet);
Also, split your code in multiple lines as #Nate Barbettini advises in comment. This will allow to inspect results in debug mode and to better understand what is going on.