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 http://stackoverflow.com/questions/1521028/rails-complex-form-validations.