MongoMapper set method return value is not same in development and production environment

Yesterday,I was informed that there is a error in my Rails App,so I run rails server in development environment and found nothing,but error still exist on server. Is the return value not the same in development and production environment? Let’s do a test! rails c development

Model.set({:id => ids}, :foo => bar) => 120
#Which class is Fixnum

rails c production

Model.set({:id => ids}, :foo => bar) => {“updatedExisting”=>true, “n”=>2, “lastOp”=>seconds: 1380198531, increment: 48, “connectionId”=>117691, “err”=>nil, “ok”=>1.0}
#Which class is BSON::OrderedHash

with the return value, we could see the difference!i dont know why and didnt look into official documents.