Migrate documents from S3 to local storage

To migrate documents stored in ActiveStorage from S3 to a storage in the local file system, do the following:

Configure a local service in config/storage.yml:

local:
  service: Disk
  root: <%= Rails.root.join("storage") %>

Run the following script to download all files from S3 and store them in your local file system. Note that at while this runs, you should not create new documents as those would otherwise be missing.

def migrate(from, to)
  configs = Rails.configuration.active_storage.service_configurations
  from_service = ActiveStorage::Service.configure from, configs
  to_service   = ActiveStorage::Service.configure to, configs
 
  ActiveStorage::Blob.service = from_service
 
  puts "#{ActiveStorage::Blob.count} Blobs to go..."
  ActiveStorage::Blob.find_each do |blob|
    print '.'
    blob.open do |tf|
      checksum = blob.checksum
      to_service.upload(blob.key, tf, checksum: checksum)
    end
  end
end
 
migrate(:amazon, :local)

Script taken from https://www.stefanwienert.de/blog/2018/11/05/active-storage-migrate-between-providers-from-local-to-amazon/

Change the storage service in config/production.rb to local and re-deploy:
```
config.active_storage.service = :local
```
You can now re-enable creation of new documents.