UserSchema.statics.authenticate = (email, password, callback) => {

    User.findOne({ email: email },(error, user) => {
            if (error) {
                return callback(error);
            } else if ( !user ) {
                console.log(user);
                let err = new Error('User not found.');
                err.status = 401;
                return callback(err);
            }
            const hash = crypto.createHash('sha1');
            let hash_password = hash.update(password).digest('hex');

            if (hash_password === password){
                console.log(user);
                return callback(null, user);
            }
            else {
                return callback();
            }
        });
};