You don't need the subset line here. Go ahead and delete it but check out these two links if you want to understand it.

https://github.com/axsh/wakame-vdc/blob/master/dcmgr/lib/dcmgr/models/instance.rb#L37

http://sequel.jeremyevans.net/rdoc/classes/Sequel/Model/ClassMethods.html#method-i-subset