The LINQ expression cannot be translated when using null check for nested projections

[Ben555555]

I'm using projection in EF Core with Linqkit to reuse expressions with Invoke(). I want to apply filters to projected models:

public class TransportModel
{
        public Guid Id { get; set; }
        public InmateModel Inmate  { get; set; }
}

public class InmateModel
{
        public Guid Id { get; set; }
        public List<Guid> InmateIds  { get; set; }
}

    public static Expression<Func<Transport, Inmate>> GetInmateProjection()
    {
        return transport => transport.Inmate;
    }

    public static Expression<Func<Inmate, InmateModel>> InmateModelProjection()
    {
        return inmate => new InmateModel()
        {
            Id = inmate.Id,
            InmateIds = inmate.Collusions.Select(x => x.InmateId).ToList()
        };
    }

    public static Expression<Func<Transport, TransportModel>> GetTransportModelProjection()
    {
        return transport => new TransportModel()
        {
            Inmate = InmateModelProjection().Invoke(GetInmateProjection().Invoke(transport)),
        };
    }

// Actual query
IQueryable<TransportModel> query = dbContext.Transports
                    .Select(GetTransportModelProjection());

This code works unless there is an Inmate which is null. Then I get the following exception:

System.InvalidOperationException: Nullable object must have a value.
at lambda_method1483(Closure , QueryContext , DbDataReader ,
ResultContext , SingleQueryResultCoordinator ) at
Microsoft.EntityFrameworkCore.Query.Internal.SingleQueryingEnumerable1.Enumerator.MoveNext() at System.Collections.Generic.List1..ctor(IEnumerable1 collection) at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source)

So I add a null check to prevent this error:

public static Expression<Func<Transport, TransportModel>> GetTransportModelProjection()
{
    return transport => transport != null ? new TransportModel()
    {
        Inmate = InmateModelProjection().Invoke(GetInmateProjection().Invoke(transport)),
    } : null;
}

The exception does not appear anymore, however I cannot filter all data anymore:

// This works
query = query.Where(transport => transport.Inmate.Id == someId);

// This does not work anymore due to null check
query = query.Where(transport => transport.Inmate.InmateIds.Contains(someId));

This is the exception I get for the second filter:

The LINQ expression could not be translated. Either rewrite the query
in a form that can be translated, or switch to client evaluation
explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable',
'ToList', or 'ToListAsync'.

Is there a bug in the translator?

The only workaround I can use so far is to flatten the models:

public static Expression<Func<Transport, TransportModel>> GetTransportModelProjection()
    {
        return transport => new TransportModel()
        {
            Inmate = InmateModelProjection().Invoke(GetInmateProjection().Invoke(transport)),
InmateIds = GetInmateProjection().Invoke(transport).Collusions.Select(x => x.Id).ToList()
        };
    }

and query like this

query = query.Where(transport => transport.InmateIds.Contains(someId));

or build the filter directly on the unprojected query:

query = dbContext.Transports.Where(transport => GetInmateProjection().Invoke(transport).InmateIds.Contains(someId));

But it would be nice to do this with nested projections.

Include provider and version information

EF Core version: 6.0.7
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: .NET 6.0
Operating system: Windows 11
IDE: Microsoft Visual Studio Enterprise 2022 (64-bit), Version 17.3.2

[ajcvickers]

This code works unless there is an Inmate which is null.

Can you post the SQL that is generated when the code works?

[Ben555555]

SELECT [t].[Id], [i].[Id], [c].[InmateId]
FROM [Transport] AS [t]
LEFT JOIN [Inmate] AS [i] ON [t].[InmateId] = [i].[Id]
LEFT JOIN [Collusion] AS [c] ON [i].[Id] = [c].[InmateId]
ORDER BY [t].[Id], [i].[Id], [c].[InmateId]

This is the generated SQL query. Also the "Nullable object must have a value" exception only appears when a projected property of InmateModel is not nullable (Id). Otherwise it will just create a new model where Inmate / InmateId is null and set null for each property. But this is not what should happen, the whole Inmate property should be null instead.

[ajcvickers]

@Ben555555 This exception is expected if the type isn't nullable. Creating an instance with null values instead of not creating an instance is tracked by #22517 (comment)

[Ben555555]

@Ben555555 This exception is expected if the type isn't nullable. Creating an instance with null values instead of not creating an instance is tracked by #22517 (comment)

But will this fix the null check from not being translated correctly?