mail-addons/res_partner_company_messages/models.py

from openerp import api
from openerp import models


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

    def read(self, fields=None, load='_classic_read'):
        res = super(Partner, self).read(fields=fields, load=load)
        if fields and 'message_ids' in fields:
            for vals in res:
                partner = self.browse(vals['id'])
                if not partner.is_company:
                    continue
                domain = [('model', '=', 'res.partner'), ('res_id', 'in', [partner.id] + partner.child_ids.ids)]
                vals['message_ids'] = self.env['mail.message'].search(domain).ids
        return res