Moped::Errors::OperationFailure:
       The operation: #<Moped::Protocol::Command
         @length=77
         @request_id=49920
         @response_to=0
         @op_code=2004
         @flags=[]
         @full_collection_name="test.$cmd"
         @skip=0
         @limit=-1
         @selector={:getlasterror=>1, :safe=>true}
         @fields=nil>
       failed with error 16837: "The positional operator did not find the match needed from the query. Unexpanded update: comments.0.comments.$.votes"