我在理解Flask中的UserMixin时遇到问题

from flask_login import UserMixin

class User(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), unique=True)
    password_hash = db.Column(db.String(128))

from werkzeug.security import generate_password_hash, check_password_hash

class User(UserMixin, db.Model):
    # ...

    def set_password(self, password):
        self.password_hash = generate_password_hash(password)

    def check_password(self, password):
        return check_password_hash(self.password_hash, password)

from flask_login import LoginManager, login_user, logout_user, login_required

login_manager = LoginManager()
login_manager.init_app(app)

@login_manager.user_loader
def load_user(user_id):
    return User.query.get(int(user_id))

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        user = User.query.filter_by(username=username).first()
        if user and user.check_password(password):
            login_user(user)
            return redirect(url_for('index'))
        flash('Invalid username or password')
    return render_template('login.html')

@app.route('/logout')
@login_required
def logout():
    logout_user()
    return redirect(url_for('index'))