router.post('/login', function(req, res) {
  var username = req.body['username'];
  var pass = req.body['password'];

  db_users.check_credentials(username, pass)
    .then(function (valid) {
      if (!valid)
        throw "Invalid username or password.";
      return db_users.get_user_by_name(username);
    })
    .then(function (user) {
      // add login stuff to our session
      req.session.user_id = user.user_id;
      req.session.username = user.username;
      req.session.email = user.email;

      res.redirect('/');
    })
    .catch(function (errMsg) {
      res.render('login', {login_error: errMsg, login_username: username});
    });
});