Absolute cache_dir breaks url

[jughead]

I currently use carrierwave 0.11.2 with fog, asset_host, and absolute cache_dir:

CarrierWave.configure do |config|
  if Rails.env.test?
    config.storage = :file
  else
    config.storage = :fog
    config.fog_directory = Rails.application.secrets.s3_bucket
    config.fog_credentials = {
        provider: 'AWS',
        aws_access_key_id: Rails.application.secrets.s3_key,
        aws_secret_access_key: Rails.application.secrets.s3_secret,
        region: Rails.application.secrets.s3_region,
    }
    config.asset_host = Rails.application.secrets.amazon_cloudfront_domain
  end

  config.cache_dir = "#{Rails.root}/tmp/uploads" # To let CarrierWave work on heroku
  config.remove_previously_stored_files_after_update = false
end

Now let's see CarrierWave::Uploader::Url.
Let's imagine I have a model with a simple uploader. When the image gets uploaded the model uploader initializes cached SanitizedFile. So the path to the file is /path/to/project/tmp/uploads/bla/bla/bla/filename.jpg. According to the trimming root path code and the asset host set, the url becomes: https://<domain>/path/to/project/tmp/uploads/bla/bla/bla/filename.jpg. And this is incorrect.
I don't know the correct way to fix this. If we don't want to return url for cached/non-stored file I would raise an exception rather than return the incorrect url.

[mshibuya]

To serve cached files, they must be placed in publicly accessible directory. But you're using Heroku, so you don't have write access to {Rails.root}/public directory.

This is the case that cache storage feature is trying to solve, see the PR for usage:
#1312
(note: this feature is not included in 0.11 releases, please use 1.0.0.rc or the master branch)