Use both ActiveRecord and Mongoid in a Rails project is easy. Add mongoid in Gemfile:
gem 'mongoid', '~> 6.1.0'
mongoid gem would change your default ORM adapter from ActiveRecord to Mongoid:
If you’re not sure which default adapter is you can try:
rails g model and check the output message
So if you wanna generate a new model inherited from ActiveRecord:
rails g model Manager --orm=active_record
active_record instead of