flask web，current_user.role是None，但表角色不是None

Question

class Role(db.Model): 
    __tablename__ = 'roles' 
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), unique=True)
    default = db.Column(db.Boolean, default=False, index=True)
    permissions = db.Column(db.Integer)
    users = db.relationship('User', backref='role', lazy='dynamic')

class User(UserMixin, db.Model): 
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.Integer, unique=True, index=True)
    username = db.Column(db.String(64), unique=True, index=True)
    role_id = db.Column(db.Integer, db.ForeignKey('roles.id'))
    password_hash = db.Column(db.String(128))
    confirmed = db.Column(db.Boolean, default=False)
    def can(self, permissions):
        print(self.role) #None
        return self.role is not None and (self.role.permissions & permissions) == permissions

@main.route('/',methods=['GET', 'POST'])
def index():
    form = PostForm()
    print(current_user.role) #None
    print(current_user.can(Permission.WRITE_ARTICLES)) #False
    if current_user.can(Permission.WRITE_ARTICLES) and form.validate_on_submit():
        post = Post(body=form.body.data, author=current_user._get_current_object())
        db.session.add(post)
        return redirect(url_for('.index'))
    posts = Post.query.order_by(Post.timestamp.desc()).all() 
    return render_template('index.html', form=form, posts=posts)


sqlite> .tables
alembic_version  posts            roles            users
sqlite> select * from roles;
1|Administrator|0|255
2|Moderator|0|15
3|User|1|7

角色中有数据，但print（current_user.role）为None，当我在类User中调用can函数时，print（self.role）也是None。

我无法理解为什么self.role是None？