I have a module with:

parent_id = fields.Many2one('res.partner')
parent_child_ids = fields.Many2many(comodel_name='res.partner',
                             relation='res_partner_rel',
                             column1='child_ids',
                             column2='partner_id')

What I expect:

I added the parent_id in the partner form where in the user can customer's parent. What I want to achieve is, when I select a parent company for a particular customer - the customer will be automatically added to child_ids of the parent company (override create and write method). And when I select child_ids in the parent company, the customer's parent id field will be updated to parent company id. How to achieve that?