Do not prematurely load ActiveRecord::Base

[fatkodima]

While investigating rails/rails#47914, I identified that this gem prematurely loads ActiveRecord::Base and so the application is not properly configured at the end.

I detected a similar problem recently with the other gem - ledermann/unread#131. The inners of the problem are described in the comments to the rails/rails#46567.

[albus522]

This change doesn't actually work because it requires something external to delayed job to trigger ActiveRecord which is not required to happen. We have already tried and rolled back a different mostly working approach that had other issues.
The problem is that Rail's argument for how you are suppose to hook into ActiveRecord doesn't apply to things like delay job. We aren't augmenting ActiveRecord, we are using it.

[fatkodima]

What do you think about doing something like this?

# lib/railtie.rb
class Railtie < Rails::Railtie
  initializer "delayed_job.initialization" do
    ActiveSupport.on_load(:after_initialize) do
      require "activerecord/base"
      require "delayed/backend/active_record"
      Delayed::Worker.backend = :active_record
    end
  end
end

if defined?(Rails)
  require_relative "railtie"
else
  require "delayed/backend/active_record"
  Delayed::Worker.backend = :active_record
end

[albus522]

An initializer was the previous attempt that didn't work. There were a number of interconnected reasons that broke people's apps and we reverted.