@app.route('/login', methods=['GET', 'POST'])
def login():
    form = LoginForm()
    if form.validate_on_submit():
        user = User.query.filter_by(user_email=form.email.data).first()
        if user is not None and user.verify_password(form.password.data):
            login_user(user, form.remember_me.data)
            return redirect(request.args.get('next') or url_for('index'))
        flash('Invalid username or password.')
    return render_template('login.html', form=form)
    
@app.route('/register', methods=['GET', 'POST'])
def register():
    form = RegisterForm()
    if form.validate_on_submit():
        user = User(user_name=form.user_name.data,
                    user_email=form.user_email.data,
                    password=form.password.data)
        db.session.add(user)
        flash('You can now login')
        return redirect(request.args.get('next') or url_for('login'))
    return render_template('register.html', form=form)

current_user.is_authenticated()

@login_manager.user_loader
def load_user(user_id):
    return User.get_id(user_id)

{% block page_content %}
    Hi,
    {% if current_user.is_authenticated() %}
        {{ current_user.user_name }}
    {% else %}
        anonymous
    {% endif %}
{% endblock %}

@app.login_manager.user_loader
def get_user():
    pass

@app.login_manager.user_loader
def get_user(user_id):
    return User.get_id(user_id)