Partials in XML builder are proving to be non-trivial.

After some initial Google searching, I found the following to work, although it's not 100%

 xml.foo do
     xml.id(foo.id)
     xml.created_at(foo.created_at)
     xml.last_updated(foo.updated_at)
     foo.bars.each do |bar|
         xml << render(:partial => 'bar/_bar', :locals => { :bar => bar })
     end
 end

this will do the trick, except the XML output is not properly indented. the output looks something similar to:

<foo>
  <id>1</id>
  <created_at>sometime</created_at>
  <last_updated>sometime</last_updated>
<bar>
  ...
</bar>
<bar>
  ...
</bar>
</foo>

The <bar> element should align underneath the <last_updated> element, it is a child of <foo> like this:

<foo>
  <id>1</id>
  <created_at>sometime</created_at>
  <last_updated>sometime</last_updated>
  <bar>
    ...
  </bar>
  <bar>
    ...
  </bar>
</foo>

Works great if I copy the content from bar/_bar.xml.builder into the template, but then things just aren't DRY.