dataset中有多个表，返回XML字符串设置主从关系

<?xml version="1.0" encoding="utf-8" ?>
<string xmlns="http://tempuri.org/">
  <Getstore>
    <Goods>
      <product_no>FB22S58613</product_no>
      <product_class>女鞋</product_class>
      <SKU>
        <product_no>FB22S58613</product_no>
        <color>46</color>
        <product_size>39</product_size>
        <color1>46</color1>
        <product_class>女鞋</product_class>
        <number3>3</number3>
        <number7>0</number7>
        <number8>0</number8>
        <number>50</number>
        <number9>0</number9>
      </SKU>
      <SKU>
        <product_no>FB22S58613</product_no>
        <color>36</color>
        <product_size>39</product_size>
        <color1>36</color1>
        <product_class>女鞋</product_class>
        <number3>2</number3>
        <number7>0</number7>
        <number8>0</number8>
        <number>50</number>
        <number9>0</number9>
      </SKU>
    </Goods>
  </Getstore>
</string>

DataSet ds = api.get_xmlstore(productno,color,productsize,users,user_key);
             ds.Tables[0].TableName = "Goods";
             ds.Tables[1].TableName = "SKU";
             DataRelation relation = new DataRelation("SKU",
             ds.Tables [0].Columns["product_no"],
             ds.Tables[1].Columns["product_no"]);
             relation.Nested = true; 
             ds.Relations.Add(relation);
             ds.DataSetName = "Getstore";
             XmlDataDocument xda = new XmlDataDocument(ds);