#initializers/authorization.rb
class AR::Base
  def editable_by(user)
     user.owns?(self) || user.is_admin?
  end

  def destroyable_by(user)
    editable_by(user)
  end

  #etc...
end

#app/models/note.rb
class Note < AR::Base
  #def editable_by(user);end

  def destroyable_by(user)
     super(user) && comments.empty?
  end

  #etc...
end