active support concern

application.rb

# config/application.rb

config.autoload_paths += Dir["#{config.root}/app/models/**/"]

user_friendship.rb

# app/models/concerns/user_friendship.rb

module UserFriendship
  extend ActiveSupport::Concern

  # put scopes, validations etc in the included block
  included do
    scope :my_scope ...
  end

  # put class methods in ClassMethods
  module ClassMethods
    def my_class_method
      # ...
    end
  end

  # do not put instance methods in InstanceMethods
  def my_instance_method
    # ...
  end
end