relationship + XML


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?