ruby copy a paperclip attachment from one model to another?

This should do the trick, you could use an after_create callback if the models are associated, if not I would recommend doing it in the controller action that creates the card.

instance_of_model_one.cardimage = instance_of_model_two.avatar
instance_of_model_one.save

old_avatar = model1.avatar;
model2.avatar.create(attachment: old_avatar.attachment);
model2.save;

It worked for me.