Master website event sale seats availability psa tde

- Task: https://www.odoo.com/web?#id=9971&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.BTJ9yFYnbxBG3VzQ
Since dereferencing `partner_id` returns a recordset, it's going to coalesce everything into a new recordset so you'd probably need `self.mapped('partner_id').ids`