Projection is not working for JSON polymorphic types

[vvitkovsky]

Hello HotChocolate experts.

I would like to ask about one issue I met. I have those models:

[JsonPolymorphic(TypeDiscriminatorPropertyName = "JsonNodeType")]
[JsonDerivedType(typeof(Employee), nameof(Employee))]
[JsonDerivedType(typeof(Employer), nameof(Employer))]
public class Person
{
    public string Id { get; set; } = Guid.NewGuid().ToString();

    public int Version { get; set; } = 1;

    public string Name { get; set; } = null!;
}

public class Employee : Person
{
    public string EmployeeId { get; set; } = null!;
}

public class Employer : Person
{
    public string EmployerId { get; set; } = null!;
}

and those type definitions for GraphQL:

public class PersonType : InterfaceType<Person>
{
    protected override void Configure(IInterfaceTypeDescriptor<Person> descriptor)
    {
        descriptor?.Name("IPerson");
    }
}

public class EmployeeType : ObjectType<Employee>
{
    protected override void Configure(IObjectTypeDescriptor<Employee> descriptor)
    {
        descriptor?.Name("Employee");
    }
}

public class EmployerType : ObjectType<Employer>
{
    protected override void Configure(IObjectTypeDescriptor<Employer> descriptor)
    {
        descriptor?.Name("Employer");
    }
}

Then I use it in a query with pagination, sorting, projection and filtering:

[UseMyCache]
[UsePaging]
[UseProjection]
[UseSorting]
[UseFiltering]    
public IQueryable<Person> GetPersons([Service] PersonContext aContext)
{
    return aContext.Persons.AsQueryable();
}

If I try to perform a query, it fails with an error System.InvalidOperationException: 'Operation is not valid due to the current state of the object.'. If I remove UseProjection, it works fine, and I can retrieve objects of both types like this:

query {
  persons {
    nodes {
      id
       __typename
       ... on Employee {
          employeeId
       }
       ... on Employer {
          employerId
       }
    }
  }
}

So, my question is - is it possible to make a projection to work for JSON polymorphic types?
I am using MongoDB over Entity Framework behind, tried also to use MongoDB integration wih HotChocolate - it doesn't work either.

Best regards,
Victor