Don't change type of a relational field after assigning value to it

[trinhanhngoc]

E.g.

    def _compute_user_id(self):
        """Do not assign self.env.user as salesman for e-commerce orders
        Leave salesman empty if no salesman is specified on partner or website

        c/p of the logic in Website._prepare_sale_order_values
        """
        website_orders = self.filtered('website_id')
        super(SaleOrder, self - website_orders)._compute_user_id()
        for order in website_orders:
            if not order.user_id:
                order.user_id = order.website_id.salesperson_id or order.partner_id.parent_id.user_id.id or order.partner_id.user_id.id
                # type of order.user_id should be `res.users` instead of `res.users | int`