I get the following error after upgrading to the 1.8.4 driver:

irb(main):001:0> User.create! username: 'fred'
MONGODB (7.5ms) banjo_staging['$cmd'].create({:create=>"users"}).limit(-1)
Mongo::OperationFailure: Database command 'create' failed: unrecognized command: $query
from /app/vendor/bundle/ruby/1.9.1/gems/mongo-1.8.4/lib/mongo/db.rb:530:in `rescue in command'
from /app/vendor/bundle/ruby/1.9.1/gems/mongo-1.8.4/lib/mongo/db.rb:521:in `command'
from /app/vendor/bundle/ruby/1.9.1/gems/mongo-1.8.4/lib/mongo/db.rb:296:in `create_collection'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/collections/master.rb:41:in `initialize'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/collection.rb:127:in `new'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/collection.rb:127:in `master'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/collection.rb:98:in `insert'
from /app/vendor/bundle/ruby/1.9.1/gems/newrelic_rpm-3.5.5.38/lib/new_relic/agent/method_tracer.rb:491:in `block in insert_with_trace_ActiveRecord_klass_insert'
from /app/vendor/bundle/ruby/1.9.1/gems/newrelic_rpm-3.5.5.38/lib/new_relic/agent/method_tracer.rb:240:in `trace_execution_scoped'
from /app/vendor/bundle/ruby/1.9.1/gems/newrelic_rpm-3.5.5.38/lib/new_relic/agent/method_tracer.rb:486:in `insert_with_trace_ActiveRecord_klass_insert'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence/operations/insert.rb:27:in `block in persist'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence/insertion.rb:26:in `block (3 levels) in prepare'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:414:in `run1298955003981228075create1742471512845801379_callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:405:in `__run_callback'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:385:in `_run_create_callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:81:in `run_callbacks'
... 7 levels...
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence/insertion.rb:24:in `block in prepare'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence/insertion.rb:22:in `tap'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence/insertion.rb:22:in `prepare'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence/operations/insert.rb:26:in `persist'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence.rb:49:in `insert'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence.rb:230:in `block (2 levels) in create!'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence.rb:229:in `tap'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence.rb:229:in `block in create!'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/threaded/lifecycle.rb:173:in `_creating'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.7.1/lib/mongoid/persistence.rb:228:in `create!'
from (irb):1
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/commands/console.rb:47:in `start'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/commands/console.rb:8:in `start'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/commands.rb:41:in `<top (required)>'
from script/rails:11:in `require'