paperclipからactive storageへのデータ移行タスク ref: http://takatoshiono.hatenablog.com/entry/2018/05/13/013606

convert_to_active_storage.rake

namespace :convert_to_active_storage do
  desc 'Convert to Activestorage'
  task :run => :environment do
    conn = ActiveRecord::Base.connection.raw_connection

    Cafe.order(:id).each do |cafe|
      puts "Convert cafe id=#{cafe.id}"

      blob_params = [
        cafe.class.generate_unique_secure_token,
        cafe.image_file_name,
        cafe.image_content_type,
        cafe.image_file_size,
        checksum(cafe.image),
        cafe.updated_at.iso8601
      ]
      attachment_params = ['image', 'Cafe', cafe.id, cafe.updated_at.iso8601]

      conn.transaction do
        puts "blob: #{blob_params}"
        conn.exec_params(<<-SQL, blob_params)
          INSERT INTO active_storage_blobs (
            key, filename, content_type, metadata, byte_size, checksum, created_at
          ) VALUES ($1, $2, $3, '{}', $4, $5, $6)
        SQL

        puts "attachment: #{attachment_params}"
        conn.exec_params(<<-SQL, attachment_params)
          INSERT INTO active_storage_attachments (
            name, record_type, record_id, blob_id, created_at
          ) VALUES ($1, $2, $3, LASTVAL(), $4)
        SQL
      end
    end
  end

  private

  def checksum(image)
    body = if Rails.env.development?
      File.read(image.path)
    else
      uri = URI.parse("https:#{image.url}")
      Net::HTTP.get(uri)
    end

    Digest::MD5.base64digest(body)
  end
end

copy_images.rake

class ActiveStorage::Blob < ActiveRecord::Base
end

class ActiveStorage::Attachment < ActiveRecord::Base
  belongs_to :blob, class_name: 'ActiveStorage::Blob'
  belongs_to :record, polymorphic: true
end

namespace :copy_images do
  desc "Copy images to the Active Storage's path"
  task :run => :environment do
    ActiveStorage::Attachment.order(:id).each do |attachment|
      name = attachment.name
      source = attachment.record.send(name).path

      if Rails.env.development?
        dest_dir = File.join(
          "storage",
          attachment.blob.key.first(2),
          attachment.blob.key.first(4).last(2))
        dest = File.join(dest_dir, attachment.blob.key)

        puts "Copy #{source} to #{dest}"

        FileUtils.mkdir_p(dest_dir)
        FileUtils.cp(source, dest)
      else
        dest = attachment.blob.key

        puts "Copy #{source} to #{dest}"

        s3 = Aws::S3::Client.new(region: ENV.fetch('AWS_REGION'))
        s3.copy_object(
          bucket: ENV.fetch('S3_BUCKET_NAME'),
          copy_source: ENV.fetch('S3_BUCKET_NAME') + source,
          key: dest
        )
      end
    end
  end
end