var session = Repository.Session;
    using (var tx = session.BeginTransaction())
     {
           var viewModel = new ViewModel();
           viewModel.Cars  = session.Query<Car>();
           viewModel.Dogs = session.Query<Dog>();
           viewModel.Cats  = session.Query<Cat>();
           viewModel.Birds  = session.Query<Bird>();
            tx.Commit();
      }