I've made a mistake by sharding a GridFS chunks collection with it's '_id'.
Then i got told that, the chunks collection should be sharded with

.

I tried to runCommand({shardcollection: "xx.xx.chunks', key: {files_id: 1, n: 1}}), then mongo told me something like 'the collection has already been sharded'.
Cannot find a command to 'remove an existing sharding rule' so i drop the chunks collection, then rerun the command above, no errors.

Then, with in database 'admin', i runned 'db.printShardingStatus()'. The result showed that the sharding rules on "xx.xx.chunks" had been modified as i wish.

But when i run my script to load data into this GridFS, the same error message from pymongo raised.

I've already found a solution, quite ugly by renaming the GridFS....