respond_to do |format|
  format.html # show.html.erb
  format.xml  { render :xml => @ltest }
end

<ltest>
....
   <sub_test>
   ...
   </sub_test>
   <sub_test>
   ...
   </sub_test>
</ltest>

format.xml  { render :xml => @ltest.to_xml(:include => :sub_tests) }

<%= @ltest.to_xml :include => [ :sub_tests ]%>

<%= @ltest.to_xml(:include => {
    	:test_group => { :include => [ :user ]},
    	:sub_tests => { :include => {
    		:attachments => {},
    		:errors => {},
    		:test_bugs => {},
    	} },
     	:attachments => {},
    	:errors => {},
    	:test_bugs => {},
    	:test_nodes => { :include => {
    		:node => { :include => [ :networks ]},
    		:attachments => {},
    	}}
    } ) %>

render :xml => @record.to_xml(:include => {:groups => {:include => {:questions => {:include => :items}}}})