I'm not going when you call update update_attribute

ruby-1.9.2-head :016 > s = Shop.find "100000000000000730064437"
=> ....
ruby-1.9.2-head :017 > s.dirty_trick
=> []
ruby-1.9.2-head :018 > s.update_attribute(:dirty_trick, s.dirty_trick << "qqq")
=> true
ruby-1.9.2-head :019 > s.dirty_trick
=> ["qqq"]
ruby-1.9.2-head :020 > s = Shop.find "100000000000000730064437"
=> ...
ruby-1.9.2-head :021 > s.dirty_trick
=> []
ruby-1.9.2-head :022 >

Use gems:
Rails 3.0.7
mongo (1.3.1)
mongoid (2.0.2)
and
mongod --version
db version v1.8.0, pdfile version 4.5
Tue Jun 21 16:17:50 git version: nogitversion