Query: Support Include/ThenInclude for navigation on derived type

Is there a solution for using ThenInclude with polymorphic types?

Example model:

public abstract class BasePosition : Entity
{
    public Order Order { get; set; }
}

public class GroupType : Entity
{}

public class GroupPosition : BasePosition
{
      public GroupType GroupType { get; set; }
}

public class SalesPosition : BasePosition
{
    public GroupPosition Group { get; set; }
}

public class Order : Entity
{
    private List<BasePosition> positions;

    public ICollection<BasePosition> Positions 
    => positions ?? (positions = new List<BasePosition>());
}

Example query:

context.Set<Order>()
.Include(x => x.Positions)
    .ThenInclude<SalesPosition>(x => x.Group) // Pseudo-Code, does obviously not work
    .ThenInclude<GroupPosition>(x => x.GroupType) // Pseudo-Code, does obviously not work

AFAIK the only way this currently works is by manually querying the derived types after loading the base shape:

var o = context.Set<Order>()
.Where(o => o.Id == 1234)
.Single();

context.Set<GroupPosition>()
    .Where(x => x.Order == o)
    .Include(x => x.GroupType)
    .Load();

context.Set<SalesPosition>()
    .Where(x => x.Order == o)
    .Include(x => x.Group)
    .Load();

[maumar]

fixed in 07afd7aa330da5b6d90d518da7375d8bbf676dfd

[divega]

@smitpatel @anpete for option 2, did you try writing the lambda like this?

queryOfBase.Include((Derived e) => e.NavigationOnDerived);