find_by.rb

    ##
    #
      def self.not_found!(*args)
        options = args.extract_options!.to_options!
        ids = args
        message = (ids.blank? and !options.blank?) ? options.inspect : nil
        ids.push(nil) if ids.blank?
        e = ::Mongoid::Errors::DocumentNotFound.new(self, *ids)
        e.message.gsub!(/id\(s\).*$/, message) if message
        raise(e)
      end

      def not_found!(*args)
        self.class.not_found!(*args)
      end

    ## finders
    #
      def self.find_by(conditions)
        where(conditions).first
      end

      def self.find_by!(conditions)
        find_by(conditions) or not_found!(conditions)
      end

      def self.create_or_update(*args)
        attributes = args.shift
        conditions = args.shift || attributes

        created = false
        doc = find(:first, :conditions => conditions)

        doc ||= (
          begin
            (created = create!(attributes))
          rescue
            find(:first, :conditions => conditions) or
            (created = create!(attributes))
          end
        )

        doc.update_attributes(attributes) unless created
        doc
      end

      def self.create_or_update!(*args)
        create_or_update(*args) || not_found!(*args)
      end