[IMP] mass_mailing: use m2o_reference on mailing.trace

Let's make use of Many2oneReference as a better tool to link the model and the id.

closes odoo/odoo#70518

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/models/mailing_trace.py

@@ -67,7 +67,7 @@ class MailingTrace(models.Model):
     source_id = fields.Many2one(related='mass_mailing_id.source_id')
     # document
     model = fields.Char(string='Document model')
-    res_id = fields.Integer(string='Document ID')
+    res_id = fields.Many2oneReference(string='Document ID', model_field='model')
     # campaign / wave data
     mass_mailing_id = fields.Many2one('mailing.mailing', string='Mailing', index=True, ondelete='cascade')
     campaign_id = fields.Many2one(