Updated request after feedback

src/Umbraco.Core/Models/ITemplate.cs

@@ -10,5 +10,11 @@
         /// </summary>
         /// <returns><see cref="RenderingEngine"/></returns>
         RenderingEngine GetTypeOfRenderingEngine();
+
+        /// <summary>
+        /// Set the mastertemplate
+        /// </summary>
+        /// <param name="masterTemplate"></param>
+        void SetMasterTemplate(ITemplate masterTemplate);
     }
 }

src/Umbraco.Core/Models/Template.cs

@@ -191,5 +191,11 @@ namespace Umbraco.Core.Models
             if (Key == Guid.Empty)
                 Key = Guid.NewGuid();
         }
+
+
+        public void SetMasterTemplate(ITemplate masterTemplate)
+        {
+            MasterTemplateId = new Lazy<int>(() => {return masterTemplate.Id});
+        }
     }
-}
+}