REXML::Document#write( output=$stdout,indent=-1,transitive=false,ie_hack=false )

indent: An integer. If -1,no
indenting will be used; otherwise,the
indentation will be twice this number
of spaces,and children will be
indented an additional amount. For a
value of 3,every item will be
indented 3 more levels,or 6 more
spaces (2 * 3). Defaults to -1

一个例子：

require "rexml/document"
 
doc = REXML::Document.new "<a><b><c>TExt</c><d /></b><b><d/></b></a>"
out = ""
doc.write(out,1)
puts out

生产：

<a>
 <b>
  <c>
   TExt
  </c>
  <d/>
 </b>
 <b>
  <d/>
 </b>
</a>

编辑：Rails已经加载了REXML,因此您只需要生成新的文档,然后将漂亮的打印的XML写入一些字符串,然后将其嵌入到< pre>标签.