Project nested entity into nested view model

Question

I need to project some POCO entities from EF into their cut down view model representations for passing to a serializer. The entities in this situation are Shipment and Requisition and have a 0..1 relationship. If I don't pass view models then it'll throw a stack overflow exception while trying to serialize the cyclic references between Requisition and Shipment.

return db.ShipmentRequisitions
                .Include(c => c.Shipment)
                .Select(r => new ViewModels.Requisition
                {
                    ID = r.ID,
                    ...
                    Shipment = r.Shipment != null ? new ViewModels.Shipment
                    {
                        ID = r.Shipment.ID,
                        ...
                    } : null
                }).AsQueryable();

If the Shipment is null, then I get an exception when trying to access r.Shipment.ID. I added a null check as above and now get the following exception.

Unable to create a constant value of type 'Api.ViewModels.Shipment'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.

This strikes me as odd, as it was quite happy to create the nested ViewModels.Shipment as part of the select before I added the null check, so it must be something to do with this.

I can see some EF in the call stack so I tried it in a quick test project with just straight objects to try it with pure LINQ.

var user = users.Select(u => new ViewModel.User
                {
                    Id = u.Id,
                    Profile = u.Profile != null ? new ViewModel.Profile
                    {
                        Id = u.Profile.Id
                    } : null
                }).ToList();

To my surprise with worked. I can't quite figure out what this doesn't work in the first code sample.

UPDATE

I've discovered that if as part of a select I do something like:

.Select(r => new ViewModels.Requisition
                    {
                        ID = r.ID,
                        ShipmentDescription = r.Shipment.Description
                    })

This is effectively flattening the required Shipment properties into a single DTO. Here I would expect to get a NullReferenceExcetion if Shipment is null on any of the records when setting the Description property. It doesn't happen however. LINQ seems to suppress the exception and the ShipmentDescription comes through just as null. It saves me doing an inline is null check on Shipment for each nested property. Unfortunately this doesn't appear to extend to the same sort of thing when creating a nested object where I'm not flattening.

Same problem here: http://stackoverflow.com/questions/10904375/can-i-project-an-optional-reference-of-an-entity-into-an-optional-reference-of-t Your last example would fail btw if you use for example `ID = r.Shipment.ID` when `ID` on the left side is *not* nullable and `Shipment` is `NULL` on a record in the DB. — Slauma, Sep 19 '12 at 23:32

score 1 · Answer 1 · answered Sep 19 '12 at 23:06

You can incorporate ViewModels.Shipment objects in your ViewModels.Requisition without the null check if all properties in the Shipment class are nullable. It's a bit of a pain though, because now Shipment objects are created with null values only where there are no shipments.

You could also turn it around: select Shipment models with collections of Requisitions in them. Worth a try if it doesn't mess up the the rest of your application too much.

score 0 · Answer 2 · answered Sep 19 '12 at 15:19

0

The problem is this line:

.SingleOrDefault(r => r.ID == id);

EF is trying to convert the expression r.ID == id to SQL, and not managing.

The solution is to simply change the line to: .ToEnumerable().SingleOrDefault(r => r.ID == id);

This works since only IQueryable is converted to SQL, not IEnumerable.

answered Sep 19 '12 at 15:19

wezten

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Thanks for the answer. The SingleOrDefault isn't required. I try to do the same Select on both the method that returns an individual record and the one that returns an IQueryable ready for paging. In both cases it has the same issue. I've updated my question to reflect this. – Daniel Revell Sep 19 '12 at 15:25

Project nested entity into nested view model

2 Answers2