Following issue comes when save command excutes => @new_node.save


  Moped::Errors::OperationFailure (The operation: #<Moped::Protocol::Command
      @length=366
      @request_id=30
      @response_to=0
      @op_code=2004
      @flags=[:slave_ok]
      @full_collection_name="campus_dev.$cmd"
      @skip=0
      @limit=-1
      @selector={:aggregate=>"nodes",:pipeline=>[{"$match"=>{"parent_id"=>"51382df8851d1912b7000009","_id"=>{"$ne"=>"513f24952f1feda4bc000002"},"position"=>{"$nin"=>[nil]}}},{"$group"=>{"_id"=>"position","count"=>{"$sum"=>1},"max"=>{"$max"=>"$position"},"min"=>{"$min"=>"$position"},"sum"=>{"$sum"=>"$position"},"avg"=>{"$avg"=>"$position"}}}]}
      @fields=nil>
    Failed with error "no such cmd"):
      app/controllers/nodes_controller.rb:37:in `create'