Introduce Ancestors, GetParent

csharp/App/Backend/Database/User.cs

@@ -30,14 +30,25 @@ public partial class Db
 
     public Boolean IsParentOfChild(User parent, User child)
     {
-        return child
-              .Unfold(GetParent)
+        return Ancestors(child)
               .Any(u => u.Id == parent.Id);
     }
 
+    private IEnumerable<User> Ancestors(User child)
+    {
+        return child.Unfold(GetParent);
+    }
+
     public User? GetParent(User u)
     {
-        return GetUserById(u.ParentId);
+        return IsRoot(u)    
+             ? null 
+             : GetUserById(u.ParentId);
+    }
+
+    public static Boolean IsRoot(User u)
+    { 
+        return u.ParentId == 0; // root has ParentId 0 by definition
     }
 
     public User? GetUserByEmail(String email) => Users.FirstOrDefault(u => u.Email == email);