Rails 3 advanced queries

In one of my queries I used

......- .includes(:classes).where(:classes => { items_classes =>
{ :item_class_id => nil } }) to find unclassified items i.e., items
with 'item_class_id IS NULL' from the join table.
How could I modify the query for classified items i.e., if I want
something like '!= nil' in place of '=>nil' ?
I had tried installing gems sqeel, meta_where, but they show
combatibility issues with my rails version 3.2.5.

Thanks,
Rashila.