SQLAlchemy custom query property problem

models.py

class _ForUserQuery(BaseQuery):
    def _get_client_user_query(self):
        raise NotImplementedError

    def _get_pk_field(self):
        raise NotImplementedError

    def for_user(self, user):
        query = self._get_client_user_query()
        if not user.is_admin:
            query = query.filter_by(user_id=user.id)
        return query

    def mine(self):
        return self.for_user(current_user)

    def get_mine_or_404(self, ident, description=None):
        # TODO: check if there's a way to automatically get the pk field
        # attribute that this query object is attached to 
        obj = self.mine().filter(self._get_pk_field() == ident).one_or_none()
        if obj is None:
            abort(404, description=description)
        return obj


class _ClientsForUserQuery(_ForUserQuery):
    def _get_client_user_query(self):
        return self.join(ClientUser)

    def _get_pk_field(self):
        return Client.id

class _PagesForUserQuery(_ForUserQuery):
    def _get_client_user_query(self):
        return self.join(Client).join(ClientUser)

    def _get_pk_field(self):
        return Page.id


class ClientUser(db.Model):
    user_id = db.Column(db.Integer, db.ForeignKey("user.id"), primary_key=True)
    client_id = db.Column(db.Integer, db.ForeignKey("client.id"), primary_key=True)
    client = db.relationship("Client", back_populates="users")
    user = db.relationship("User", back_populates="clients")


class Client(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(255), unique=True, nullable=False)

    users = db.relationship("ClientUser", back_populates="client")
    pages = db.relationship("Page", back_populates="client")

    query_class = _ClientsForUserQuery


class Page(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(255), nullable=False)

    client_id = db.Column(db.Integer, db.ForeignKey("client.id"))
    client = db.relationship("Client", back_populates="pages")

    query_class = _PagesForUserQuery