Add registering of client-side relationship properties (#5936)

app/models/Notification.ts

@@ -47,7 +47,7 @@ class Notification extends Model {
   /**
    * The document that the notification is associated with.
    */
-  @Relation(() => Document)
+  @Relation(() => Document, { onDelete: "cascade" })
   document?: Document;
 
   /**
@@ -58,7 +58,7 @@ class Notification extends Model {
   /**
    * The comment that the notification is associated with.
    */
-  @Relation(() => Comment)
+  @Relation(() => Comment, { onDelete: "cascade" })
   comment?: Comment;
 
   /**