01_unfactored.rb

class Activity < ActiveRecord::Base
  # columns:
  # * entity_id
  # * entity_type
  # * action
  # * created_at

  belongs_to :entity, :polymorphic => true
end

class User
  has_many :activities, :as => :entity

  after_save :log_activity

  private

  def log_activity
    activities.create(:action => 'create')
  end
end

class Post
  has_many :activities, :as => :entity

  private

  def log_activity
    activities.create(:action => 'create')
  end
end

02_refactored_with_observer.rb

class Activity < ActiveRecord::Base
  # columns:
  # * entity_id
  # * entity_type
  # * action
  # * created_at

  belongs_to :entity, :polymorphic => true
end

class User
  has_many :activities, :as => :entity
end

class Post
  has_many :activities, :as => :entity
end

class LoggableObserver < ActiveRecord::Observer
  observe :user, :post

  def after_save(loggable)
    loggable.activities.create(:action => 'create')
  end
end

03_refactored_with_module.rb

module Loggable
  extend ActiveSupport::Concern

  included do
    after_save :log_activity
  end

  private

  def log_activity
    activities.create(:action => 'create')
  end
end

class Activity < ActiveRecord::Base
  # columns:
  # * entity_id
  # * entity_type
  # * action
  # * created_at

  belongs_to :entity, :polymorphic => true
end

class User
  include Loggable
  has_many :activities, :as => :entity
end

class Post
  include Loggable
  has_many :activities, :as => :entity
end

04_refactored_with_module_2.rb

module Loggable
  extend ActiveSupport::Concern

  included do
    after_save :log_activity
    has_many :activities, :as => :entity
  end

  private

  def log_activity
    activities.create(:action => 'create')
  end
end

class Activity < ActiveRecord::Base
  # columns:
  # * entity_id
  # * entity_type
  # * action
  # * created_at

  belongs_to :entity, :polymorphic => true
end

class User
  include Loggable
end

class Post
  include Loggable
end