Mongoid 3, 4, 5, 6 or 7.
Add the gem to your Gemfile:
gem 'delayed_job_mongoid'
Create the indexes (and don't forget to do this on your production database):
script/rails runner 'Delayed::Backend::Mongoid::Job.create_indexes'
To generate script/delayed_job:
rails generate delayed_job
That's it. Use delayed_job as normal.