Debugging MongoDB mapreduce is painful, so I'm not 100% sure I understand what's going on here, but I think I get the general idea...
The error message I'm getting is this: mr failed, removing collectionCannotCreateIndex: namespace name generated from index name "my_dbname.tmp.mr.collectionname_69.$_id.aggregation_method_1__id.date_key.start_1__id.date_key.timeres_1__id.region.center_2dsphere" is too long (127 byte max)
The key I'm using for mapreduce is a complex object with four or five properties, so I'm guessing what's happening is that when Mongo tries to create its temporary output collections using my specified key, it tries to auto-create an index on that complex key; but since the key itself has several properties, the default name for the key is too long. When I index complex objects like this under "normal" circumstances, I just give the index a custom name. But I don't see a way to do that for the collections mapreduce generates automatically.
Is there a simple way to fix this without changing my key structure?
Well, turns out I was tricked by the error message! the <collectionname> in the error message referenced above is the name of the INPUT collection whose records I'm processing with mapreduce... but the index it's referring to is an index that was part of the OUTPUT collection! So I just had to give the index in the output collection a name, and voila, problem solved. What weird behavior.