function logIn(req, res) {
    let { login, pass } = req.body;
    User.findOne({ login: login }, { pass: pass }, (err, user) => {
        if (err) return res.json(new Error(err));
        if (!user) return res.json(new Error('No such username.'));
        req.session.user = user;
        return res.status(200).send('Success.');
    });
}

function updateData(req, res) {
    let { login, pass, firstName, middleName, lastName, birthDate } = req.body;
    let updateInfo = { "$set": {
        "login": login,
        "pass": pass,
        "firstName": firstName,
        "middleName": middleName,
        "lastName": lastName,
        "birthDate": birthDate
    }}
    User.findOneAndUpdate({ login: login }, updateInfo, (err, user) => {
        if (err) res.json(new Error(err));
        if (!user) return res.json(new Error('No such username.'));
        res.json(user);
    });
}

function checkSession(req, res) {
    if (!req.session.user) return res.json(new Error('Unauthorized.'));
    return res.status(200).send('Session exists');
}