Using ::ActiveJob::QueueAdapters::AbstractAdapter breaks compatibility with rails 6

[jklimke]

Hi.

in Version 4.2 the active job adapter was included in Delayed Job itself. My Application is still using rails 6 which does not include the ::ActiveJob::QueueAdapters::AbstractAdapter that is currently the base class of the DelayedJobAdapter class that is included in delayed job 4.2.

See: https://github.com/rails/rails/tree/v6.1.7.10/activejob/lib/active_job/queue_adapters

Usage as base class here:

delayed_job/lib/active_job/queue_adapters/delayed_job_adapter.rb

Line 10 in eaef62d

class DelayedJobAdapter < ::ActiveJob::QueueAdapters::AbstractAdapter

Old version of DelayedJobAdapter in Rails 6:

https://github.com/rails/rails/blob/v6.1.7.10/activejob/lib/active_job/queue_adapters/delayed_job_adapter.rb

Possiblilty 1:

• define the base class when not defined by rails
  Possibilty 2:
• add the breaking compatibility with rails 6 to the docs

[albus522]

We explicitly do not replace the rails adapter if it exists. The only thing I can think of is that you don't have rails first in your gemfile messing up the load order.

[raldred]

Same issue.
This is caused by the in-housing of the adapter #1236

Abstract adapter doesn't existing in Rails 6.
I believe we can restore support for Rails 6 similar to this approach with sidekiq when similar happened there.
sidekiq/sidekiq#6749

I have opened a PR
#1249

You can get support in the meantime adding the following to your application.rb
After requiring rails.
Before Bundler.require

require 'rails/all'

# FIXME: delayed_job 4.2.0 ships its own delayed_job_adapter.rb which inherits from
# AbstractAdapter, which exists from Rails 7+.
# Stub it here to restore support for now
# https://github.com/collectiveidea/delayed_job/issues/1246).
module ActiveJob
  module QueueAdapters
    AbstractAdapter = Class.new unless defined?(AbstractAdapter)
  end
end

Bundler.require(*Rails.groups)