@api.model
    def update_shoes_order(self,):

            datas = safe_eval(self.datas)
            domain = [('quatation_id', '=', self.id)]
            shoes = self.env['shoes.order'].search(domain, limit=1)

            vals = {
                'partner_id': self.partner_id.id,
                'user_id': self.user_id.id,
                'amount_total': datas['amount_total'],
            }

            for line in datas['lines']:
                line = line and line[2]
                vals = {
                    u'product_id': line.get('product_id'),
                    u'amount_total': line.get('price'),

                }

            shoes.service_ids.write(vals)

In shoes.order i have service ids lines. the goal is to update those line from pos.quotation. but with this code, it only overwrites existing lines with new one. how can i add additional line rather than overwrite?