I was trying to figure out the same thing by looking at our prod to see if I could find out the current index name. That's when I noticed that the version seemed wrong. The doc on ensure_index made it sound like indices are fairly short-lived (http://api.mongodb.org/python/current/api/pymongo/collection.html#pymongo.collection.Collection.ensure_index) "When an index is created (or ensured) by PyMongo it is “remembered” for cache_for seconds. "

The error I got was: 
```
common/lib/xmodule/xmodule/modulestore/mongo/base.py line 299 in __init__
      zip(('_id.' + field for field in Location._fields), repeat(1)),
    /Users/dmitchell/mitx_all/python/lib/python2.7/site-packages/pymongo/collection.py line 914 in ensure_index
      return self.create_index(key_or_list, cache_for, **kwargs)
    /Users/dmitchell/mitx_all/python/lib/python2.7/site-packages/pymongo/collection.py line 821 in create_index
      **self._get_wc_override())
    /Users/dmitchell/mitx_all/python/lib/python2.7/site-packages/pymongo/collection.py line 359 in insert
      continue_on_error, self.__uuid_subtype), safe)
    /Users/dmitchell/mitx_all/python/lib/python2.7/site-packages/pymongo/mongo_client.py line 852 in _send_message
      rv = self.__check_response_to_last_error(response)
    /Users/dmitchell/mitx_all/python/lib/python2.7/site-packages/pymongo/mongo_client.py line 795 in __check_response_to_last_error
      raise OperationFailure(details["err"], details["code"])
   OperationFailure: ns name too long, max size is 128

   -------------------- >> begin captured stdout << ---------------------
 index_name =  _id.tag_1__id.org_1__id.course_1__id.category_1__id.name_1__id.revision_1
```

When I set the index name, the error went away.

I'll look again to see how to query prod for the list of indices.