How to Migrate from Paperclip to Rails ActiveStorage

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• Source code for this episode

db/migrate/convert_to_active_storage.rb

Dir[Rails.root.join("app/models/**/*.rb")].sort.each { |file| require file }

class ConvertToActiveStorage < ActiveRecord::Migration[5.2]
  require 'open-uri'

  def up
    # postgres
    # get_blob_id = 'LASTVAL()'
    # mysql / mariadb
    # get_blob_id = 'LAST_INSERT_ID()'
    # sqlite
    get_blob_id = 'LAST_INSERT_ROWID()'

    active_storage_blob_statement = ActiveRecord::Base.connection.raw_connection.prepare(<<-SQL)
      INSERT INTO active_storage_blobs (
        key, filename, content_type, metadata, byte_size,
        checksum, created_at
      ) VALUES (?, ?, ?, '{}', ?, ?, ?)
    SQL

    active_storage_attachment_statement = ActiveRecord::Base.connection.raw_connection.prepare(<<-SQL)
      INSERT INTO active_storage_attachments (
        name, record_type, record_id, blob_id, created_at
      ) VALUES (?, ?, ?, #{get_blob_id}, ?)
    SQL

    models = ActiveRecord::Base.descendants.reject(&:abstract_class?)

    transaction do
      models.each do |model|
        attachments = model.column_names.map do |c|
          if c =~ /(.+)_file_name$/
            $1
          end
        end.compact

        model.find_each.each do |instance|
          attachments.each do |attachment|
            if instance.send(attachment).exists?
              active_storage_blob_statement.execute(
                key(instance, attachment),
                instance.send("#{attachment}_file_name"),
                instance.send("#{attachment}_content_type"),
                instance.send("#{attachment}_file_size"),
                checksum(instance.send(attachment)),
                instance.updated_at.iso8601
              )

              active_storage_attachment_statement.
                execute(attachment, model.name, instance.id, instance.updated_at.iso8601)
            end
          end
        end
      end
    end

    active_storage_attachment_statement.close
    active_storage_blob_statement.close
  end

  def down
    raise ActiveRecord::IrreversibleMigration
  end

  private

  def key(instance, attachment)
    # SecureRandom.uuid
    # Alternatively:
    filename = instance.send("#{attachment}_file_name")
    klass = instance.class.table_name
    id = instance.id
    id_partition = ("%09d".freeze % id).scan(/\d{3}/).join("/".freeze)

    "#{klass}/#{attachment.pluralize}/#{id_partition}/original/#{filename}"
  end

  def checksum(attachment)
    # local files stored on disk:
    #url = attachment.path
    #Digest::MD5.base64digest(File.read(url))

    # remote files stored on another person's computer:
    url = attachment.url
    Digest::MD5.base64digest(Net::HTTP.get(URI(url)))
  end
end

lib/tasks/paperclip.rake

namespace :paperclip do
  task migrate: :environment do
    klass = User
    attachment = 'avatar'
    name_field = :"#{attachment}_file_name"

    klass.where.not(name_field => nil).find_each do |instance|
      # This step helps us catch any attachments we might have uploaded that
      # don't have an explicit file extension in the filename

      filename = instance.send("#{attachment}_file_name")

      next if filename.blank?

      id = instance.id
      id_partition = ("%09d".freeze % id).scan(/\d{3}/).join("/".freeze)

      url = "https://nyc3.digitaloceanspaces.com/gorails/#{klass.table_name}/#{attachment.pluralize}/#{id_partition}/original/#{filename}"

      instance.send(attachment.to_sym).attach(
        io: open(url),
        filename: instance.send(name_field),
        content_type: instance.send(:"#{attachment}_content_type")
      )
    end
  end
end