Rails after_initialize only on "new"

Putting the logic in the controller could be the best answer as you stated, but you could get the after_initialize to work by doing the following:

after_initialize :add_productdef add_product  self.product ||= Product.newend

That way, it only sets product if no product exists. It may not be worth the overhead and/or be less clear than having the logic in the controller.

Edit: Per Ryan's answer, performance-wise the following would likely be better:

after_initialize :add_productdef add_product  self.product ||= Product.new if self.new_record?end

Surely after_initialize :add_product, if: :new_record? is the cleanest way here.

Keep the conditional out of the add_product function

If you do self.product ||= Product.new it will still search for a product every time you do a find because it needs to check to see if it is nil or not. As a result it will not do any eager loading. In order to do this only when a new record is created you could simply check if it is a new record before setting the product.

after_initialize :add_productdef add_product  self.product ||= Product.new if self.new_record?end

I did some basic benchmarking and checking if self.new_record? doesn't seem to affect performance in any noticeable way.