make deletion easyer when linked with other objects and make gameboard url into a function

dbschema/default.esdl

@@ -1,13 +1,17 @@
 module default {
+  function is_winner_me(winner: str) -> bool
+    using (winner = "me");
+
+  function gameboard_url(id: uuid) -> str
+    using ("https://play.battlesnake.com/game/" ++ <str>id);
 
   type GameBoard {
     overloaded required id: uuid {
       readonly := true;
       constraint exclusive;
     }
-    url: str {
-      readonly := true;
-    }
+    url := gameboard_url(.id);
+
     required created_at: datetime {
       readonly := true;
     }
@@ -16,12 +20,15 @@ module default {
     }
     required map: str {
       readonly := true;
+      default := "standard";
     }
     required single type: GameType {
       readonly := true;
+      on source delete delete target if orphan;
     }
     required single ruleset: Ruleset {
       readonly := true;
+      on source delete delete target if orphan;
     }
     required winner: str {
       readonly := true;
@@ -32,7 +39,10 @@ module default {
     }
     required single snake: Snake {
       readonly := true;
+      on source delete delete target if orphan;
     }
+
+    is_winner_me := is_winner_me(.winner)
   }
 
   type GameType {