cakephp一对多自定义关系连接

I have an Order table which has many IPN. However, I am not using cakephp conventions because the IPN table is from Paypal. I want to join the order table's order_num field to the IPN table's custom field. So it would be like: select * from orders left join ipn on orders.order_num = ipn.customI have an Order table which has many IPN. Howev