def each(&blk)
      fields.each do |field|
        if field.child?
          val = self.send(field.reader)
          yield val unless val.nil?
        end
      end
      return self
    end
def each(&blk)
      fields.each do |field|
        if field.child?
          val = self.send(field.reader)
          yield val unless val.nil?
        end
      end
      return self
    end
Yield each child in field order.