# autoload concerns
module YourApp
  class Application < Rails::Application
    config.autoload_paths += %W(
      #{config.root}/app/controllers/concerns
      #{config.root}/app/models/concerns
    )
  end
end

# app/models/concerns/trashable.rb
module Trashable
  extend ActiveSupport::Concern
  
  included do
    default_scope   where(trashed: false)
    scope :trashed, where(trashed: true)
  end
  
  def trash
    update_attribute :trashed, true
  end
end

# app/models/message.rb
class Message < ActiveRecord::Base
  include Trashable, Subscribable, Commentable, Eventable
end