Shard keys are supposed to be immutable. This is enforced during update when using an update modifier like $set but not when passing an entire document:

mongos> db.runCommand({shardcollection: 'foo.things', key:

{'foobar': 1}

, unique: true})

{ "collectionsharded" : "foo.things", "ok" : 1 }

...
mongos> db.things.find()

{ "_id" : ObjectId("4e7d176c3c3472e653777dce"), "foobar" : 123 }

mongos> db.things.update(

{'foobar': 123}

, {'$set': {'foobar': 456}})
Can't modify shard key's value fieldfoobar for collection: foo.things
mongos> doc = db.things.findOne()

{ "_id" : ObjectId("4e7d176c3c3472e653777dce"), "foobar" : 123 }

mongos> doc.foobar = 456
456
mongos> db.things.save(doc)
mongos> db.things.find()

{ "_id" : ObjectId("4e7d176c3c3472e653777dce"), "foobar" : 456 }

See the attached js for an example using update directly (without upsert).