How to correctly add data to a table via SQLALCHEMY many-to-many?

Question

Flask

Good afternoon.
There is such a trivial model:

ItemTable = Table('ItemTable', Base.metadata,
    Column('post_id', Integer, ForeignKey('post.id')),
    Column('tags_id', Integer, ForeignKey('tag.id')))

class Post(Base):
    __tablename__ = 'post'
    id = Column(Integer, primary_key = True)
    title = Column(Text)
    body = Column(Text)
    timestamp = Column(DateTime, default=datetime.datetime.utcnow())
    user_id = Column(Integer, ForeignKey(User.id))
    tags = relationship('Tag', secondary=ItemTable, backref='post')

    def __init__(self,title=None,body=None):
        self.title = title
        self.body = body

    def __repr__(self):
        return '<Post %r>' % (self.title)


class Tag (Base):
    __tablename__ = 'tag'
    id = Column (Integer, primary_key = True)
    title = Column (Text)
    posts = relationship ('Post', secondary=ItemTable, backref='tag')

    def __init__(self,title=None):
        self.title = title


    def __repr__(self):
        return '<Tag %r>' % (self.title)

Let's say I add a post with multiple tags. How to do it right?

class AddPost(BaseView):
    def is_accessible(self):
        return g.user.is_authenticated()
    @expose('/', methods=('GET', 'POST'))
    def addpost_view(self):
        if not g.user.is_authenticated():
            return redirect(url_for('AdminView.login_view'))
        form = PostForm()
        if helpers.validate_form_on_submit(form):
            p=Post(form.title.data, form.text.data)
            p.tags.append(form.tags.data.split(','))
            db_session.add(p)
            db_session.commit()