Problem:
Cannot update a shard_key value even if the document would still reside on the same shard.

Fails with "change would move shards!"

Reproduce:
Have setup a 2 shard system, as follows

> db.printShardingStatus()
— Sharding Status —
sharding version:

{ "_id" : 1, "version" : 3 }

shards:

{ "_id" : "shard0000", "host" : "10.194.251.95:27000" } { "_id" : "shard0001", "host" : "10.202.251.67:27000" }

databases:

{ "_id" : "admin", "partitioned" : false, "primary" : "config" } { "_id" : "scaleout", "partitioned" : true, "primary" : "shard0000" }

scaleout.blogs chunks:
{ "ts" :

{ $minKey : 1 }

} -->>

{ "ts" : 0 }

on : shard0000

{ "t" : 2000, "i" : 1 }

{ "ts" : 0 }

-->> { "ts" :

{ $maxKey : 1 }

} on : shard0001

{ "t" : 2000, "i" : 0 }

I then inserted the following

{ "_id" : ObjectId("4cab5334e9d125376ee1006d"), "author" : "jim", "clocktime" : "Tue Oct 05 2010 12:32:52 GMT-0400 (EDT)", "ts" : 1, "tags" : [ "yes", "no" ] }

This document should reside on shard001 based on the output above. However the following fails

> db.blogs.update(

{ ts : 1}

,

{ts : 2}

)
change would move shards!

Solution:
Either the error message is wrong or we should support shard_key updates within the same shard.