app.post('/login', (req, res) => {
  // hash the password
  hash(req.password, (err, hashedPassword) => {
    if (err) {
      throw new Error(err.message);
    }

    // find combination of username and password in db
    User.find({ username: req.username, password: hashedPassword }, (err, user) => {
      if (err) {
        throw new Error(err.message);
      }

      // save session to redis
      redis.hget(user.id, (err, session) => {
        if (err) {
          throw new Error(err.message);
        }

        // load user information after session is succesfully saved
        Transaction.find({ userId: user.id }, (err, transaction) => {
          return transaction;
        });
      });
    });
  });
});