Now we don't have to specify the field names on products, yay!
I tried desparately to get it working for the nested variant forms too, but sadly the form builder refuses to acknowledge the relationship. The form builder simply doesn't support a collection of objects in this way.
We could try creating a fake model similar to ProductSet that accepts_nested_attributes_for :products. But it woudl be better to create a custom form builder to do it.
Or, just manually specify field names for now!