Book.delete_all(:title => "The Great Gatsby")

module Mongoid
  module Persistence
    module ClassMethods
      alias :delete_all_old :delete_all
      
      def delete_all(conditions = nil)
        selector = (conditions || {})[:conditions] || {}
        selector.merge!(:_type => name) if hereditary?

        if collection.find(selector).count == collection.count
          raise "Are you crazy?!?! Did you forget :conditions?"
        end

        delete_all_old(conditions)
      end
    end
  end
end