Create active storage attachment records from paperclip files in PostgreSQL

convert_to_active_storage.rb

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

  def change

    active_storage_blob_statement = ActiveRecord::Base.connection.raw_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 = ActiveRecord::Base.connection.raw_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, #{"(SELECT max(id) from active_storage_blobs)"}, $4)
    SQL

    Rails.application.eager_load!
    models = ActiveRecord::Base.descendants.reject(&:abstract_class?) - [Apartment::Adapters::AbstractAdapter::SeparateDbConnectionHandler]

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

        if attachments.blank?
          next
        end

        model.find_each.each do |instance|
          attachments.each do |attachment|
            
            if instance.send(attachment).exists? then 
              if instance.send(attachment).path.blank?
                next
              end
              ActiveRecord::Base.connection.raw_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
              ])

              ActiveRecord::Base.connection.raw_connection.exec_prepared(
              'active_storage_attachment_statement', [
                attachment,
                model.name,
                instance.id,
                instance.updated_at.iso8601,
              ])
            end
          end
        end
      end
    end
    ActiveRecord::Base.connection.execute('DEALLOCATE PREPARE active_storage_attachment_statement')
    ActiveRecord::Base.connection.execute('DEALLOCATE PREPARE active_storage_blob_statement')
  end

  private

  def key(instance, attachment)
    SecureRandom.uuid
  end

  def checksum(attachment)
    url = attachment.path
    Digest::MD5.base64digest(File.read(url))

  end
end