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# 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)
  end

  module ClassMethods
    def trashed
      unscoped.where(trashed: true)
    end
  end

  module InstanceMethods
    def trash
      update_attribute :trashed, true
    end
  end
end

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