OpenNebula

[] operator is returning nil when the XML element has more levels inside.

A exists? or hash_element? method should be implemented for checking the existence of an element instead of getting a nil from the [][ operator.

if vm['TEMPLATE/DISK'] != nil
vm.each('TEMPLATE/DISK') do |disk|

Now you have to ignore the if

vm.each('TEMPLATE/DISK') do |disk|

because vm['TEMPLATE/DISK'] returns nil:

Solution:
if vm.hash_element?('TEMPLATE/DISK')
vm.each('TEMPLATE/DISK') do |disk|