Carrierwave + RMagick - how to upload generated Image? (no form)

I have a article_set, which has many set_items, each set item has an
image. I want to create an collage of the images, so one image out of
many, and save it with carrierwave.

for this, I have a method called "create_collage" inside my
article_sets.rb model, which fires on after_create.

see this gist with the code

It works that he generates the image and puts it in the temp folder,
but whatever I try to upload it from the model through carrierwave,
nothing gets uploaded and the :blog_image attribute stays NULL.

Any ideas how to tell the uploader to work off that file?

thank you!