Migrate from plain ol' delayed_job to ActiveJob without losing jobs in the queue when you deploy

delayed_job_active_job_compat.rb

# Allow legacy jobs in the queue to be initialized and processed now that they use ActiveJob
#
# Each job must also define `#init_with(coder)` to define how serialized instance variables are
# used to initialize the new ActiveJob instance.
module Delayed::LegacyJob
  class Proxy < SimpleDelegator
    # ActiveJob calls `#perform_now` internally when executing a job to pass arguments to `#perform`
    def perform
      perform_now
    end
  end

  def payload_object
    object = super
    object.is_a?(ActiveJob::Base) ? Proxy.new(object) : object
  end
end

# Allow deserializing legacy jobs that used to be a struct and are now a class
module Delayed::PsychExt::StructToObject
  def visit_Psych_Nodes_Mapping(object)
    if %r{^!ruby/struct:(.+)$} =~ object.tag
      klass = resolve_class(Regexp.last_match[1]) rescue nil
      return revive(klass, object) if klass
    end

    super(object)
  end
end

module Delayed::ActiveJobInitWith
  # Hook into DelayedJob/Psych's deserialization method to map `#perform` method
  # parameters from instance variables of legacy jobs.
  def init_with(coder)
    initialize *method(:perform).parameters.map { |_, name| coder[name.to_s] }
  end
end

Delayed::Job.include Delayed::LegacyJob
Delayed::PsychExt::ToRuby.include Delayed::PsychExt::StructToObject
ActiveJob::Base.include Delayed::ActiveJobInitWith