my_gem.rb file store in lib/my_gem.rb

my_gem.rb

require 'my_gem/core_ext'
module MyGem
  module Whateva
    extend ActiveSupport::Concern
    
    included do
      puts "Woohoo!"
    end
    
    module ClassMethods
      def cm
        "Class Methods"
      end
    end
    
    module InstanceMethods
      def im
        "Instance Methods"
      end
    end
    
  end
end

ActiveRecord::Base.send :include, MyGem::Whateva