Hi,

it is better to avoid using the standard field parent_id which is of the many2one type. There is logic which relies upon that, and you would spent quite much time to recover it.

It is better to introduce 2 new fields of many2many:

contact_m2m_ids = fields.Many2many(
    "res.partner",
    "rel_table",
    "direct_rel_id",
    "back_rel_id",
    string="Contacts"
)
parents_m2m_ids = fields.Many2many(
    "res.partner",
    "rel_table",
    "back_rel_id",
    "direct_rel_id",   
    string="Companies"
)

Basically, many2many fields are based on a intermediary table, which might be used for both direct and backward relation. In the example it is the table 'rel_table', which has 2 columns 'back_rel_id' & 'direct_rel_id'. It is possible to avoid defining 'rel_table': but if you have 2 relational fields for the same model, it will not work. For more info look at: https://www.odoo.com/documentation/10.0/reference/orm.html#relational-fields

If you want to use 'parent_id' anyway, then it is better to make a computed m2m field, which would use it but would not redefine it:

@api.multi
@api.depends("parent_id")
def _compute_m2m_parents_ids(self):
    for partner in self:
        if partner.parent_id:
             partner.m2m_parents_ids = [(6, 0, [partner.parent_id.id])]
        else:
             partner.m2m_parents_ids = [(6, 0, [])]            

m2m_parents_ids = fields.Many2many(   
    "res.partner",    
    "rel_table",    
    "back_rel_id",    
    "direct_rel_id",       
    string="Companies",
    compute=_compute_m2m_parents_ids,
    store=True,
)