Generate ActiveStorage image variants after upload

I have a model that has a photo attached and this photo has few variants.
I would like to display many of these photos as a gallery and i am using one of the variants as a thumbnail and another one as a light box representation. Pretty straightforward.

I noticed that this variants are generated on first request, which can take quite some time for the first user as there can be many photos in a single gallery.

Is there a way to generate variants after the model has been successfully saved?
Preferably in background.

I was searching around and could't find any resources on this, but maybe i was not persistent enough :)
Anyway, I would appreciate any tips on this.

Hey Mitja, this post is a bit old now but maybe I can help the next person looking for this information :)

The way I've dealt with this exact same situation was with a job (ActiveJob or Sidekiq or whatever you choose to use). On the model that has the photo attachment, I have a callback like this:

after_commit :process_images, if: :photos_attached?

process_images simply calls a job: ProcessImagesJob.perform_later(self)

ProcessImagesJob looks something like this. I removed my app specific code, but hopefully you'll get the idea:

photos.each do |photo|
   next unless photo&.representable?

    preview_variant = photo.representation(resize_to_limit: [300, 300])
    large_variant = photo.representation(resize_to_limit: [1000, 1000])

    preview_variant.processed
    large_variant.processed
end

This generates the variant files on my storage provider, so they're ready to be used when the user views the image. Hope that helps!