EF 6 Fluent Configuration - recursively nested objects with parent - TPH

Question

I have the following entity which has a collection of itself and a parent property which I want to be null if it is the root widget:

public class CanvasWidgetQuery
{
    public int CanvasWidgetId { get; set; }

    public int? ParentCanvasWidgetId { get; set; }
    public virtual CanvasWidgetQuery ParentCanvasWidget { get; set; }
    public virtual ICollection<CanvasWidgetQuery> InnerCanvasWidgets { get; set; }

}

Because CanvasWidgetQuery is actually just a base class (might as well be abstract but I'm not putting abstract until I see it work without it). I wanted to create a generic mapping configuration to map derived types of CanvasWidgetQuery:

public class CanvasWidgetQueryMap<TCanvasWidgetQuery> : EntityTypeConfiguration<TCanvasWidgetQuery>
    where TCanvasWidgetQuery : CanvasWidgetQuery
{
    public CanvasWidgetQueryMap() 
    {
        this.HasKey(q => q.CanvasWidgetId);

        this.HasMany(w => w.InnerCanvasWidgets)
            .WithOptional(w => w.ParentCanvasWidget)
            .HasForeignKey(w => w.ParentCanvasWidgetId);

    }
}

Regardless of the derived type, they all have those properties but unfortunately it complains about the WithOptional call (2 errors at once):

 Error  1   Cannot implicitly convert type 'CanvasWidgetQuery' to 'TCanvasWidgetQuery'. An explicit conversion exists (are you missing a cast?) 


 Error  2   Cannot convert lambda expression to delegate type 'System.Func<CanvasWidgetQuery,TCanvasWidgetQuery>' because some of the return types in the block are not implicitly convertible to the delegate return type  

If I move this up a level to the mapping of the actual derived type:

 public class SearchCanvasWidgetQueryMap : CanvasWidgetQueryMap<SearchCanvasWidgetQuery>
{
    public SearchCanvasWidgetQueryMap() 
    {
        this.HasMany(w => w.InnerCanvasWidgets)
            .WithOptional(w => w.ParentCanvasWidget)
            .HasForeignKey(w => w.ParentCanvasWidgetId);
    }
}

Now it complains that it cannot implicity convert CanvasWidgetQuery into SearchCanvasWidgetQuery meanwhile:

public class SearchCanvasWidgetQuery : CanvasWidgetQuery
{

}

How can I accomplish what I'm after

Answer 1

Have you tried defining the map for the base class which holds the properties which define the hierarchy?

e.g:

public class CanvasWidgetQueryMap<CanvasWidgetQuery> : EntityTypeConfiguration<CanvasWidgetQuery>
{
    public CanvasWidgetQueryMap() 
    {
        this.HasKey(q => q.CanvasWidgetId);

        this.HasMany(w => w.InnerCanvasWidgets)
            .WithOptional(w => w.ParentCanvasWidget)
            .HasForeignKey(w => w.ParentCanvasWidgetId);

    }
}

There's some good additional information in the answer here: CTP 4 doesn't consider base class properties which might be helpful if the above doesn't achieve what you're after.