Currently Mongoid implements special handling for serialization of ActiveSupport::TimeWithZone objects to BSON, since this class does not derive from Time. Driver and bson library itself don't handle these objects specially. As a result, when querying data on a driver collection in a mongoid app (vs using a model) the query built is not correct.

good_time = Time.now
bad_time = Time.now.in_time_zone("Pacific Time (US & Canada)")
User.collection.find(:u_at => {:$lt => bad_time}).to_a.length # this query will be wrong

# the problem is that:
good_time.to_bson != bad_time.to_bson

A patch has been proposed in https://github.com/mongodb/bson-ruby/pull/104, however it couples bson-ruby too tightly to AS. My preference is to have a separate file for AS integration which Mongoid would require. The test suite in bson-ruby should also be run separately with and without having AS loaded.

Lastly, code handling AS::TWZ in Mongoid should be examined to see if any of it should be moved to bson-ruby/driver or deleted, if driver/bson-ruby handle the serialization.