i am trying to create a models in a single .py file, after saved file, restart server and trying to upgrade app, it is showing error as i have used Many2one fields to have below info (country,state & city) in another model, (that model was present and now i am going to add fields country_id, state_id, city_id as Many2one using below models), when go to check Models through Settings -> Technical it is not showing any of these models in below .py file. what i am doing wrong or missing something? please help.

from odoo import models, fields, api


class Countries(models.Model):
    _name = 'hp.countries'
    _description = "Countries"

    name = fields.Text(string="Country Name", required=True)
    slug = fields.Char(string="Slug")

    country_ids = fields.One2many('hp.states', 'country_id')


class States(models.Model):
    _name = 'hp.states'
    _description = "States"

    name = fields.Text(string="State Name", required=True)
    country_id = fields.Many2one('hp.countries', "Country", required=True)

    state_ids = fields.One2many('hp.cities', 'state_id')


class Cities(models.Model):
    _name = 'hp.cities'
    _description = "Cities"

    name = fields.Text(string="City Name", required=True)
    country_id = fields.Many2one('hp.countries', "Country", required=True)
    state_id = fields.Many2one('hp.states', "State", required=True, domain="[('country_id', '=', country_id)]")

regards