EF Core 1-1 relationship with automatic discriminator

public sealed class CommentsThread
{
    public long Id { get; init; }
    //...
}
public interface ICommentable
{
    long CommentsThreadId { get; set; }
    CommentsThread CommentsThread { get; set; }
}
public sealed Blogpost : ICommentable
{
    public long CommentsThreadId { get; set; }
    public CommentsThread CommentsThread { get; set; }
}

string type;
if (thread.BlogpostId is not null) type = "blogpost";
else if (thread.ProfileId is not null) type = "profile";
else if (thread.DocumentId is not null) type = "document";
else type = "unknown";

public CommentsSource Source { get; private set; }
public long SourceId { get; private set; }