Moped::Errors::OperationFailure failed with error "no such cmd - ruby-on-rails

Question

I recently upgraded from mongoid 2.0.2 to mongoid 3 with rails 3.2.12 and ruby 1.9.3 .
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'

Answer 1

You didn't mention also upgrading MongoDB version to the latest (at that time).
If you were pointing at an older MongoDB server that didn't recognize the "aggregate" command, then you would get exactly this error.
All instances of similar errors seem to have been pointing at older mongod process.