From 8935353e38291541b7e82648e23c00dd5dd3294c Mon Sep 17 00:00:00 2001
From: ig <ig ig@devnullmail.com>
Date: Thu, 9 Mar 2023 11:12:56 +0100
Subject: [PATCH] Introduce Ancestors, GetParent

---
 csharp/App/Backend/Database/User.cs | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/csharp/App/Backend/Database/User.cs b/csharp/App/Backend/Database/User.cs
index 3782a36d0..081a162a2 100644
--- a/csharp/App/Backend/Database/User.cs
+++ b/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);