Migration for upgrading existing delayed_job records when moving from Rails 4 to Rails 5.

migrate_dj_to_rails5.rb

require 'ruby-progressbar'

class MigrateDjToRails5 < ActiveRecord::Migration[5.2]
  include MigrationHelper

  def up
    execute create_backup('delayed_jobs')
    migrator = DelayedJobMigrator.new(Delayed::Job.all)
    migrator.migrate
  end

  def down
    execute copy_field_from_backup('delayed_jobs', 'handler')
    execute load_dropped_records_from_backup('delayed_jobs')
  end

  class DelayedJobMigrator
    def initialize(jobs = Delayed::Job.all)
      @exceptions = []
      @corrected_job_ids = []
      @deleted_job_ids = []
      @jobs = jobs
      @progress_bar = ProgressBar.create(total: jobs.count, format: "%t: |%w| %e")
    end

    def migrate_job(job)
      begin
        data = YAML.load_dj(job.handler)
      rescue
        begin
          job.payload_object = TempToRuby.create.accept(Psych.parse(job.handler))
        rescue => exception
          if exception.message =~ /Couldn't find (.+) with 'id'=/
            # job is no longer valid
            @deleted_job_ids << job.id
            job.delete
          end       
          @exceptions << exception
          return
        end
        job.save
        @corrected_job_ids << job.id
      end
    end

    def migrate
      @jobs.find_each do |job|
        migrate_job(job)
        @progress_bar.increment
      end

      @corrected_job_ids.each do |id|
        puts "Corrected job_id: #{id}"
      end

      @exceptions.each do |exception|
        puts "Exceptions:: #{exception}"
      end

      puts "#{@corrected_job_ids.count} jobs corrected"
      puts "#{@exceptions.count} exceptions encountered (should be same as jobs deleted)"
      puts "#{@deleted_job_ids.count} jobs deleted"
    end

    class TempToRuby < Delayed::PsychExt::ToRuby
      def visit_Psych_Nodes_Mapping(object)
        if %r{^!ruby/object:ActiveRecord::AttributeSet}.match(object.tag.to_s)
          {}
        elsif %r{^!ruby/object:(.+)$}.match(object.tag.to_s)
          klass = resolve_class(Regexp.last_match[1])
          if klass < ActiveRecord::Base
            payload = Hash[*object.children.map { |c| accept c }]
            return super unless payload['raw_attributes']
            id = payload['raw_attributes'][klass.primary_key]
            klass.unscoped.find(id)
          else
            super
          end
        else
          super
        end
      end
    end
  end
end

migration_helper.rb

# Helper module for commonly used migration related methods
module MigrationHelper
  def create_backup(table_name, suffix='_copy')
    table_name_copy = table_name + suffix

    <<-SQL
      DROP TABLE IF EXISTS #{table_name_copy};

      CREATE TABLE #{table_name_copy} AS TABLE #{table_name};
    SQL
  end

  def copy_field_from_backup(table_name, field, suffix='_copy')
    table_name_copy = table_name + suffix
    records_to_update = <<-SQL
      UPDATE #{table_name}
      SET #{field} = #{table_name_copy}.#{field}
      FROM #{table_name_copy}
      WHERE #{table_name_copy}.id = #{table_name}.id
    SQL
  end

  def load_dropped_records_from_backup(table_name, suffix='_copy')
    table_name_copy = table_name + suffix

    <<-SQL
      INSERT INTO #{table_name}
      SELECT #{table_name_copy}.* FROM #{table_name_copy} 
      LEFT JOIN #{table_name} on #{table_name}.id = #{table_name_copy}.id
      where #{table_name}.id is null
    SQL
  end
end