Complex form validation problem

I have a complex form for my Work model. It accepts nested attributes for Credits, Images and Videos. (it is a modified version of Eloy Duran's complex form example)

I want to validate presence of at least one Credit I want to validate presence of at least one Image or one Video

When I do (in work.rb): validates_presence_of :credits It validates properly when I try to submit the form without any credits but it does not validate if I remove the last credit (by checking a check_box that adds "_delete" to credit's attributes). It submits the work deleting the one and only credit leaving the work without any credits.

If I could find a way to check if last credit is selected for deletion from the Work model I coult prevent the save but feels like the wrong way, trying to access credits attributes from the work model.

Do you have any idea on how I can validate these properly?

Thanks

I have got this answered on Rails Complex Form Validations? - Stack Overflow.