hi all iam a newbie to odoo , what iam trying to achieve is i want to prevent from adding duplicate products in order line , if the same exits already we should not able to add the duplicate product  , I tried like this but its not working 

@ api.constrains ('order_line') 

    def _check_exist_product_in_line (self): 

        product_ids = self.env ['product.product']. search ([('default_code', '=', 'KIB_PRF')]) 

        for line in self.order_line:

          # self.env ['sale.order.line']. create ({'product_id': product_id.id, 'order_id': self.id})

          if line.product_id in product_ids: 

            raise ValidationError (_ ('Product already added.'))

          # product_id.append (line.product_id.id)