concerning.rb

class Todo < ActiveRecord::Base
  concerning :EventTracking do
    included do
      has_many :events
    end

    def latest_event
      ...
    end

    private

    def some_internal_method
      ...
    end
  end
end

ruby.rb

class Todo < ActiveRecord::Base
  module EventTracking
    def self.included(base)
      base.has_many :events
    end

    def latest_event
      ...
    end

    private

    def some_internal_method
      ...
    end
  end

  include EventTracking
end