Requirement as defined by customer:

1. We have collection that contains all documents that are to be processed.
2. At point of time, we will consider this collection as archive, and we will go ahead and create a new collection. But the documents pending processing in previous collection needs to be moved over to new collection. However in current releases, we don't have mechanism to move the documents in a sharded environment in backend itself. (One of the request that we have asked for).
3. So, under these conditions, it will be useful to do a map reduce on more than one collection.