gistfile1.rb

  def prepare(user)
    can :manage, :all do |action, object_class, object|
      # Check first to see if the object is public
      if (object.has_attribute?("public") and object.public)
        true 
      elsif(user.present?)
        user.permissions.find_all_by_action(action).any? do |permission|
          permission.object_type.constantize == object_class &&
          (object.nil? || permission.object_id.nil? || permission.object_id == object.id)
        end
      end
    end
  end