I have two models, product and order. Product - cost - id Order - cost - product_id Each time someone places an order, it captures the product_id through a radio button value in the "new order" form. In the controller when creating the new order it needs to set order.cost to order.product.cost. Logically I thought the code should be something like this: def create ... @order.cost == @order.product.cost ... end However I can't seem to make it work at all, hence me asking the question here. Any help is answering (or naming) the question would be greatly appreciated.