force singleordefault to execute - asp.net-mvc-4

Question

Can anyone tell me why the following code does not work in an api controller (MVC4)? I get an error saying that the "Cannot access a disposed object". I found that in other cases there should be a call to ToList() so that it forces the query to be executed. But in my case, I'm using SingleOrDefault(), isn't this executed on the database right away?
public HttpResponseMessage GetPurchaseOrder(int POId)
{
PurchaseOrder po;
HttpResponseMessage resp;
using (PMDataDataContext database = new PMDataDataContext())
{
po = database.PurchaseOrders.SingleOrDefault(x => x.POId == POId);
if (po == null)
{
var message = string.Format("id = {0} not found", POId);
HttpError err = new HttpError(message);
resp = Request.CreateResponse(HttpStatusCode.NotFound, err);
}
else
{
resp = Request.CreateResponse(HttpStatusCode.OK, po);
}
}
return resp;
}

Answer 1

Yes, but if you have any lazy-loaded properties in PurchaseOrder, when Web API tries to serialize the object, EF will try to fetch the data from the database and for this the context will be needed but it will not be available.

Answer 2

Try it so that the context to database gets disposed right after the query. SingleOrDefault should send the query to the database immediately.
public HttpResponseMessage GetPurchaseOrder(int POId)
{
PurchaseOrder po;
HttpResponseMessage resp;
using (PMDataDataContext database = new PMDataDataContext())
{
po = database.PurchaseOrders.SingleOrDefault(x => x.POId == POId);
}
if (po == null)
{
var message = string.Format("id = {0} not found", POId);
HttpError err = new HttpError(message);
resp = Request.CreateResponse(HttpStatusCode.NotFound, err);
}
else
{
resp = Request.CreateResponse(HttpStatusCode.OK, po);
}
return resp;
}

Answer 3

Badri led me to solve my problem. When I placed a breakpoint in my code and followed in debug mode, it seemed that one of the associated objects needed the context to be fetched. The property PurchaseOrder.Item.ItemCategory was giving an ObjectDisposedException, when I was looking at the related properties.
I solved it by placing database.DeferredLoadingEnabled = false right before fetching the po using the SingleOrDefault call.