IQueryable<T> with EntityObject using Generics & I

I have a search repository for EntityFramework 4.0 using LinqKit with the following search function:

public IQueryable<T> Search<T>(Expression<Func<T, bool>> predicate) 
    where T : EntityObject
{
    return _unitOfWork.ObjectSet<T>().AsExpandable().Where(predicate);
}

And another class which uses the IQueryable return value to subset the query in ways that are not possible using the Boolean LinqKit PredicateBuilder expressions:

public IQueryable<T> SubsetByUser<T>(IQueryable<T> set, User user) 
    where T : EntityObject
{
    return set.Join(_searcher.Search<Metadatum>((o) => o.UserGUID == user.GUID),
                    arc => arc.GUID,
                    meta => meta.ElementGUID,
                    (arc, meta) => arc);
}

The problem here is that 'T' as EntityObject does not define GUID, so I can't use this. The natural response to this is to actually define the SubsetByUser() method to use a constraint with a GUID property:

public IQueryable<T> SubsetByUser<T>(IQueryable<T> set, User user) 
    where T : IHaveMetadata
{
    return set.Join(_searcher.Search<Metadatum>((o) => o.UserGUID == user.GUID),
                    arc => arc.GUID,
                    meta => meta.ElementGUID,
                    (arc, meta) => arc);
}

But this doesn't work. I'm using LinqKit and the Expandable() method results in:

System.NotSupportedException: Unable to cast the type 'Oasis.DataModel.Arc' to
type 'Oasis.DataModel.Interfaces.IHaveMetadata'. LINQ to Entities only supports 
casting Entity Data Model primitive types

I need an IQueryable to be returned. I can do a fake like this:

public IQueryable<T> SubsetByUser<T>(IQueryable<T> set, User user) 
    where T : EntityObject
{
    return set.AsEnumerable()
              .Join(_searcher.Search<Metadatum>((o) => o.UserGUID == user.GUID),
                    arc => arc.GUID,
                    meta => meta.ElementGUID,
                    (arc, meta) => arc)
              .AsQueryable();
}

Which, of course, works, but which is also, of course, a bat-shit crazy thing to do. (The whole reason I want IQueryable is to not execute the query until we're final.

I've even tried this:

public IQueryable<T> SubsetByUser<T>(IQueryable<T> set, User user) 
    where T : EntityObject
{
    return set.Join(_searcher.Search<Metadatum>((o) => o.UserGUID == user.GUID),
                    arc => arc.GetType().GetProperty("GUID").GetValue(arc,null),
                    meta => meta.ElementGUID,
                    (arc, meta) => arc);
}

Which uses reflection to get the Runs collection– working around the compiler error. I thought that was rather clever, but it results in the LINQ exception:

System.NotSupportedException: LINQ to Entities does not recognize the 
method 'System.Object GetValue(System.Object, System.Object[])' method, 
and this method cannot be translated into a store expression.

I could also try to change the search method:

public IQueryable<T> Search<T>(Expression<Func<T, bool>> predicate) 
    where T : IRunElement
{
    return _unitOfWork.ObjectSet<T>().AsExpandable().Where(predicate);
}

But this won't compile of course because IRunElement is not an EntityObject, and ObjectSet constrains T as a class.

The final possibility is simply making all the parameters and return values IEnumerable:

public IEnumerable<T> SubsetByUser<T>(IEnumerable<T> set, User user) 
    where T : EntityObject
{
    return set.Join(_searcher.Search<Metadatum>((o) => o.UserGUID == user.GUID),
                    arc => arc.GetType().GetProperty("GUID").GetValue(arc,null),
                    meta => meta.ElementGUID,
                    (arc, meta) => arc);
}

Which also works, but which, again, doesn't allow us to delay instantiation until the end.

So, it seems like there's little I can do to make this work without instantiating everything as an IEnumerable and then returning it using AsQueryable(). Is there some way I can put this together that I'm missing?

The natural response to this is to actually define the SubsetByUser() method to use a constraint with a GUID property: ... But this doesn't work. I'm using LinqKit and the Expandable() method results in: System.NotSupportedException: Unable to cast the type 'Oasis.DataModel.Arc' to type 'Oasis.DataModel.Interfaces.IHaveMetadata'. LINQ to Entities only supports casting Entity Data Model primitive types

You're very close with that. You can make this work if you use an ExpressionVisitor that removes all unnecessary casts (casts to a base type or an implemented interface) that are automatically generated.

public IQueryable<T> SubsetByUser<T>(IQueryable<T> set, User user) 
    where T : IHaveMetadata
{
    Expression<Func<T, Guid>> GetGUID = arc => arc.GUID;
    GetGUID = (Expression<Func<T, Guid>>)RemoveUnnecessaryConversions.Instance.Visit(GetGUID);
    return set.Join(_searcher.Search<Metadatum>((o) => o.UserGUID == user.GUID),
        GetGUID,
        meta => meta.ElementGUID,
        (arc, meta) => arc);
}

public class RemoveUnnecessaryConversions : ExpressionVisitor
{
    public static readonly RemoveUnnecessaryConversions Instance = new RemoveUnnecessaryConversions();

    protected RemoveUnnecessaryConversions() { }

    protected override Expression VisitUnary(UnaryExpression node)
    {
        if (node.NodeType == ExpressionType.Convert
            && node.Type.IsAssignableFrom(node.Operand.Type))
        {
            return base.Visit(node.Operand);
        }
        return base.VisitUnary(node);
    }
}

Alternatively, create the expression tree manually using the Expression.* functions, so that you can avoid including the cast in the first place.