Check this out. It may solve your issue. HTH.
[QueryInterceptor("Entities")]
public Expression<Func<Entity, bool>> EntitiesFilter()
{
var user = HttpContext.Current.User.Identity.Name;
if (string.IsNullOrEmpty(user))
return (Entity e) => false;
else if (user== "Administrator")
return (Entity e) => true;
else
return (Entity e) => e.Username == user;
}