social/mail_activity_partner/models/mail_activity.py

# Copyright 2018 Eficent Business and IT Consulting Services, S.L.
# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
from odoo import api, fields, models


class MailActivity(models.Model):
    _inherit = "mail.activity"

    partner_id = fields.Many2one(
        comodel_name="res.partner",
        index=True,
        compute="_compute_res_partner_id",
        store=True,
    )

    commercial_partner_id = fields.Many2one(
        related="partner_id.commercial_partner_id",
        string="Commercial Entity",
        store=True,
        related_sudo=True,
        readonly=True,
    )

    @api.depends("res_model", "res_id")
    def _compute_res_partner_id(self):
        for obj in self:
            res_model = obj.res_model
            res_id = obj.res_id
            if res_model == "res.partner":
                obj.partner_id = res_id
            else:
                res_model_id = obj.env[res_model].search([("id", "=", res_id)])
                if "partner_id" in res_model_id._fields and res_model_id.partner_id:
                    obj.partner_id = res_model_id.partner_id
                else:
                    obj.partner_id = None