Ruby on rails Rails3XMLBuilder：没有数据时不返回XML元素

当字段中有数据时，我会得到这种漂亮的XML：

<DescriptiveDetail>
  <ProductComposition>00</ProductComposition>
  <ProductForm>01</ProductForm>
  <Measure>
    <MeasureType>01</MeasureType>
    <MeasureType>198</MeasureType>
    <MeasureType>mm</MeasureType>
  </Measure>
</DescriptiveDetail>

---

您是否尝试检查@isbnlist是否为空，如果为空，请使用条件语句迭代所有元素，或者只打印简短/空版本？

谢谢您的思考。问题是，@isbnlist中的一些属性会有数据，而有些则不会@isbnlist是Isbn.allWell那么，另一种方法是在每一行检查属性是否有数据：xml.ProductIDType productcode.idtype，除非productcode.idtype.nil？要更新它，必须调用blank？而不是零？阻止标签返回。无返回但为空？什么也不退。非常感谢！这是纯Rails的力量！

<DescriptiveDetail>
  <ProductComposition>00</ProductComposition>
</DescriptiveDetail>

<DescriptiveDetail>
  <ProductComposition></ProductComposition>
  <ProductForm></ProductForm>
  <Measure>
    <MeasureType></MeasureType>
    <MeasureType></MeasureType>
    <MeasureType></MeasureType>
  </Measure>
</DescriptiveDetail>

<Measure/>

xml.instruct!(:xml, :encoding => "ISO-8859-1")
xml.ONIXMessage(:release=>"3.0") do
  xml.Header do
    xml.Sender
      xml.SenderName 
  end
  @isbnlist.each do |isbn|
    xml.Product do
      xml.RecordReference isbn.id
      xml.NotificationType isbn.notificationtype
        isbn.productcodes.each do |productcode|
          xml.ProductIdentifier do 
            xml.ProductIDType productcode.idtype
            xml.IDValue productcode.idvalue
          end
        end
      xml.DescriptiveDetail do
        xml.ProductComposition isbn.descriptivedetail_productcomposition_productcomposition
        xml.ProductForm isbn.descriptivedetail_productcomposition_productform
          isbn.measurements.each do |measurement|
            xml.Measure do
              xml.MeasureType measurement.descriptivedetail_measure_measuretype 
              xml.MeasureType measurement.descriptivedetail_measure_measurement 
              xml.MeasureType measurement.descriptivedetail_measure_measureunitcode
            end 
          end  
        xml.TitleDetail do
          xml.TitleType isbn.descriptivedetail_titledetail_titletype
            xml.TitleElement do
              xml.TitleElementLevel isbn.descriptivedetail_titledetail_titleelement_titleelementlevel
              xml.TitlePrefix isbn.descriptivedetail_titledetail_titleelement_titleprefix
              xml.TitleWithoutPrefix isbn.descriptivedetail_titledetail_titleelement_titlewithoutprefix
            end 
          xml 
        end 
        end 
      end
    end
  end