I need to change partner_id in purchase.order based on another selectio field :

class PurchaseOrder(models.Model):
    _inherit = 'purchase.order'
    po_type = fields.Selection([('internal', 'داخلية'), ('external', 'خارجية')], string="نوع المشتريات", required=1,
                               default='internal', readonly=check_foreign_group)
    @api.onchange('po_type')
    def _onchange_partner_id(self):
        if self.po_type=='internal':
            domain = [('is_foreign', '=', False)]

        else:
            domain=[('is_foreign', '=', True)]
        return domain

    partner_id = fields.Many2one(domain=_onchange_partner_id)

i do that but it doses not work