I'm trying to create new many2many field inside Contact model. I have this code inside models.py

# -*- coding: utf-8 -*-

from odoo import models, fields, api

class custom_partner_fields(models.Model):
    _inherit = 'res.partner'

    company_ids = fields.Many2many(comodel_name='res.company' string='Related user\'s Allowed companies', readonly=True)

But it does not work and no field is created. When I add another test field like this:

class custom_partner_fields(models.Model):
    _inherit = 'res.partner'

    company_ids = fields.Many2many(comodel_name='res.company' string='Related user\'s Allowed companies', readonly=True)
    test = fields.Char(string='Test field')

It isn't created as well. I works only when I comment the "company_ids".

I'm also having trouble with figuring out how to assign values to that field in other way than automatic action. Can I use realted field to update it every time when the source record changes? How do I do this? I couldn't find any examples.