How to use the ihatemoney.models.Bill function in ihatemoney

def delete(self, project, bill_id):
        bill = Bill.query.delete(project, bill_id)
        db.session.commit()
        if not bill:
            return "Not Found", 404
        return "OK", 200

def add_bill():
    form = get_billform_for(g.project)
    if request.method == "POST":
        if form.validate():
            # save last selected payer in session
            session["last_selected_payer"] = form.payer.data
            session.update()

            bill = Bill()
            db.session.add(form.save(bill, g.project))
            db.session.commit()

            flash(_("The bill has been added"))

            args = {}
            if form.submit2.data:
                args["add_bill"] = True

            return redirect(url_for(".list_bills", **args))

    return render_template("add_bill.html", form=form)

def post(self, project):
        form = get_billform_for(project, True, meta={"csrf": False})
        if form.validate():
            bill = Bill()
            form.save(bill, project)
            db.session.add(bill)
            db.session.commit()
            return bill.id, 201
        return form.errors, 400

def get_bills(self):
        """Return the list of bills related to this project"""
        return (
            Bill.query.join(Person, Project)
            .filter(Bill.payer_id == Person.id)
            .filter(Person.project_id == Project.id)
            .filter(Project.id == self.id)
            .order_by(Bill.date.desc())
            .order_by(Bill.creation_date.desc())
            .order_by(Bill.id.desc())
        )

def get_member_bills(self, member_id):
        """Return the list of bills related to a specific member"""
        return (
            Bill.query.join(Person, Project)
            .filter(Bill.payer_id == Person.id)
            .filter(Person.project_id == Project.id)
            .filter(Person.id == member_id)
            .filter(Project.id == self.id)
            .order_by(Bill.date.desc())
            .order_by(Bill.id.desc())
        )

def get(self, project, id):
            try:
                return (
                    self.join(Person, Project)
                    .filter(Bill.payer_id == Person.id)
                    .filter(Person.project_id == Project.id)
                    .filter(Project.id == project.id)
                    .filter(Bill.id == id)
                    .one()
                )
            except orm.exc.NoResultFound:
                return None

def list_bills():
    bill_form = get_billform_for(g.project)
    # set the last selected payer as default choice if exists
    if "last_selected_payer" in session:
        bill_form.payer.data = session["last_selected_payer"]
    # Preload the "owers" relationship for all bills
    bills = g.project.get_bills().options(orm.subqueryload(Bill.owers))

    return render_template(
        "list_bills.html",
        bills=bills,
        member_form=MemberForm(g.project),
        bill_form=bill_form,
        add_bill=request.values.get("add_bill", False),
        current_view="list_bills",
    )