I upgraded mongoid to version 3.0.21 and I got this error on this query:

def delete_dependent_notifications
notifications = Notification.any_of(

{subject_id: Moped::BSON::ObjectId.from_string(self.id) , subject_type: self.class}

,

{secondary_subject_id: Moped::BSON::ObjectId.from_string(self.id) , secondary_subject_type: self.class}

)
notifications.delete_all
end

Query:
MOPED: 127.0.0.1:27017 QUERY database=trust_the_crowd_development collection=notifications selector={"$query"=>{"$or"=>[

{"subject_id"=>"5113ee1be1054eb29b000007", "subject_type"=>"Comment"}

,

{"secondary_subject_id"=>"5113ee1be1054eb29b000007", "secondary_subject_type"=>"Comment"}

]}, "$orderby"=>{"created_at"=>-1}}

Processing by CommentsController#destroy as JS
2013-02-08T02:53:13+00:00 app[web.1]: Parameters:

{"question_id"=>"test-of-video-with-question-yet-again", "id"=>"5114688b046fa61f02000002"}

2013-02-08T02:53:13+00:00 app[web.1]: Completed 500 Internal Server Error in 52ms
2013-02-08T02:53:14+00:00 heroku[router]: at=info method=DELETE path=/questions/test-of-video-with-question-yet-again/comments/5114688b046fa61f02000002 host=staging.trustthecrowd.com fwd=190.244.143.70 dyno=web.1 queue=0 wait=0ms connect=5ms service=295ms status=500 bytes=2967
2013-02-08T02:53:14+00:00 app[web.1]:
2013-02-08T02:53:14+00:00 app[web.1]: Moped::Errors::OperationFailure (The operation: #<Moped::Protocol::Command
2013-02-08T02:53:14+00:00 app[web.1]: @length=85
2013-02-08T02:53:14+00:00 app[web.1]: @request_id=176
2013-02-08T02:53:14+00:00 app[web.1]: @response_to=0
2013-02-08T02:53:14+00:00 app[web.1]: @op_code=2004
2013-02-08T02:53:14+00:00 app[web.1]: @flags=[]
2013-02-08T02:53:14+00:00 app[web.1]: @full_collection_name="trustthecrowd-staging.$cmd"
2013-02-08T02:53:14+00:00 app[web.1]: @skip=0
2013-02-08T02:53:14+00:00 app[web.1]: @limit=-1
2013-02-08T02:53:14+00:00 app[web.1]: @selector={:getlasterror=>1, :safe=>true}
2013-02-08T02:53:14+00:00 app[web.1]: @fields=nil>
2013-02-08T02:53:14+00:00 app[web.1]: failed with error 10068: "invalid operator: $or"
2013-02-08T02:53:14+00:00 app[web.1]:
2013-02-08T02:53:14+00:00 app[web.1]: See https://github.com/mongodb/mongo/blob/master/docs/errors.md
2013-02-08T02:53:14+00:00 app[web.1]: for details about this error.):
2013-02-08T02:53:14+00:00 app[web.1]: app/models/comment.rb:80:in `delete_dependent_notifications'
2013-02-08T02:53:14+00:00 app[web.1]: app/controllers/comments_controller.rb:37:in `destroy'
2013-02-08T02:53:14+00:00 app[web.1]: lib/middleware/canonical_host.rb:11:in `call'

But there is no error when I run this query from Mongo console:

db.notifications.remove({$or:[

{"subject_id": ObjectId("5112fc6519bfad3e55000033"), "subject_type": "Comment"}

,

{"secondary_subject_id": ObjectId("507a08258099ca0002000807"), "secondary_subject_type": "Comment"}

]})

Thanks.
Aldo.