Postgres Paperclip to ActiveStorage Rake Task

backfill_active_storage.rake

namespace :backfill do
  task active_storage: :environment do
    require 'open-uri'

    def key(instance, attachment)
      SecureRandom.uuid
      # Alternatively:
      # instance.send("#{attachment}_file_name")
    end

    def checksum(attachment)
      # local files stored on disk:
      url = attachment.path

      begin
        Digest::MD5.base64digest(File.read(url))
      rescue StandardError
        "fakechecksum"
      end

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

    ### START SCRIPT
    
    # postgres
    get_blob_id = 'LASTVAL()'
    connection = ActiveRecord::Base.connection.raw_connection

    active_storage_blob_statement = connection.prepare('active_storage_blob_statement', <<-SQL)
      INSERT INTO active_storage_blobs (
        "key", filename, content_type, metadata, byte_size, checksum, created_at
      ) VALUES ($1, $2, $3, '{}', $4, $5, $6)
    SQL

    active_storage_attachment_statement = connection.prepare('active_storage_attachment_statement', <<-SQL)
      INSERT INTO active_storage_attachments (
        name, record_type, record_id, blob_id, created_at
      ) VALUES ($1, $2, $3, #{get_blob_id}, $4)
    SQL

#    models = [ Payment, User, etc]
    Rails.application.eager_load!
    models = ActiveRecord::Base.descendants.reject(&:abstract_class?)

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

        next if attachments.blank?

        model.find_each.each do |instance|
          attachments.each do |attachment|
            next if instance.send(attachment).path.blank?

            connection.exec_prepared(
              'active_storage_blob_statement',
              [
                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
              ]
            )

            connection.exec_prepared(
              'active_storage_attachment_statement',
              [
                attachment, 
                model.name, 
                instance.id, 
                instance.updated_at.iso8601
              ]
            )
          end
        end
      end
    end

    #active_storage_attachment_statement.close
    #active_storage_blob_statement.close
  end
end

Thanks for this; I've turned it into a PR: thoughtbot/paperclip#2613 .