router.post('/', (req, res, next) => {
    const id = req.session.user_id,
        password = req.body.password,
        t1 = req.body.t1,
        t2 = req.body.t2;

    if (!userId) res.send({ error: 'user_id required for login' });

    getUserById(id)
        .then(user => {
            if (!user) res.send({ error: `No user found with userId ${id}` });
            if (password) user.password = password;

            const rec_1 = getFromTable(1, t1);
            const rec_2 = getFromTable(2, t2);
            const promises = [rec_1, rec_2];

            Promise.all(promises).then(() => {
                if (user.col1 !== rec_1._id) user.col1 = rec_1._id;
                if (user.col2 !== rec_2._id) user.col2 = rec_2._id;

                user.save(err => {
                    if (err) res.send({ err });
                    res.redirect('/settings');
                });
            });
        });
});

/**
 * Given an ID, retrieve a user from the DB.
 * @param {number} id - A user ID.
 */
function getUserById(id) {
    return new Promise((resolve, reject) => {
        mongoose.model('User').findById(id, (err, user) => {
            if (err) reject(err);
            resolve(user);
        });
    });
}

/**
 * Given a table ID and a name, retrieve a "rec" from the DB.
 * @param {number} table - A table ID.
 * @param {string} name - A name.
 */
function getFromTable(table, name) {
    return new Promise((resolve, reject) => {
        mongoose.model(`Table${table}`).findOne({ name }, (err, rec) => {
            if (err) reject(err);
            resolve(rec);
        });
    });
}