ignore current source setting, at serialisation

src/isn/Settings.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations.Schema;
 using System.Linq;
+using Newtonsoft.Json;
 
 namespace isn
 {
@@ -64,13 +66,15 @@ namespace isn
 
         string currentSourceKey;
 
+        [JsonIgnore, NotMapped]
         public string CurrentSourceKey
         {
             get {
                 return currentSourceKey;
             }
             set{
-                if (!Sources.ContainsKey(value)) throw new InvalidOperationException($"source is invalid ({value})");
+                if (!Sources.ContainsKey(value)) 
+                throw new InvalidOperationException($"source is invalid ({value})");
                 currentSourceKey = value;
             }
         }