In the mongo shell, after connecting directly to a secondary, I can "use local", and then save documents to that database. I know they're not replicated, and that I need to copy the result somewhere else if I want to preserve it.

However, if I try to use the "out" option of "mapreduce" to write the output of a map-reduce job to "local", I get an error that I can't do that. It looks like the write is checked too high up in the call stack. It looks like that comes from here: https://github.com/mongodb/mongo/blob/master/db/commands/mr.cpp#L984-991 .