関心事の分離とアンチパターン ref: https://qiita.com/yhirano55/items/92ba558bcb19f3a1f233

file0.rb

class Todo < ApplicationRecord
  # Other todo implementation
  # ...

  ## Event tracking
  has_many :events

  before_create :track_creation
  after_destroy :track_deletion

  private
    def track_creation
      # ...
    end
end

file1.rb

class Todo < ApplicationRecord
  # Other todo implementation
  # ...

  module EventTracking
    extend ActiveSupport::Concern

    included do
      has_many :events
      before_create :track_creation
      after_destroy :track_deletion
    end

    private
      def track_creation
        # ...
      end
  end
  include EventTracking
end

file2.rb

class Todo < ApplicationRecord
  # Other todo implementation
  # ...

  include TodoEventTracking
end

file3.rb

class Todo
  # Other todo implementation
  # ...

  concerning :EventTracking do
    included do
      has_many :events
      before_create :track_creation
      after_destroy :track_deletion
    end

    private
      def track_creation
        # ...
      end
  end
end

Todo.ancestors
# => Todo, Todo::EventTracking, Object

file4.rb

class Module
  module Concerning
    # Define a new concern and mix it in.
    def concerning(topic, &block)
      include concern(topic, &block)
    end

    def concern(topic, &module_definition)
      const_set topic, Module.new {
        extend ::ActiveSupport::Concern
        module_eval(&module_definition)
      }
    end
  end
  include Concerning
end