relationship + XML

Hey,

I am developping a website in ruby on rails and I am wondering how i
can creaty something like this!

xml:
<activity>
  <..>
  <user></user>
   <plaze></plaze>
</activity>

1 parent model activity and 2 child models user and plaze!

How can i make this in ruby on rails? How do i have to make the
relationship and how i can get the xml reponse like this?

Thank you

nobody? I probably have to work with nested resources but how do i get
XML like this?