findAndModify throws exception when _id is in the update document and "new":true

Even though an exception is thrown, the document is still inserted.

This was generated from http://groups.google.com/group/mongodb-user/browse_thread/thread/00623298f8c04836

To Reproduce in 2.0.1: (This is reproducible in 1.8.4 as well, although the outputs are slightly different. )

> db.test.drop()
true
> db.test.save({_id:1, "name" : "Object 1"})
> db.runCommand({findAndModify:"test", "query" :

{ "name" : "asdf" }

, "update" :

{ "_id" : 2, "name" : "Object 2" }

, "upsert" : true } )
{
"lastErrorObject" :

{ "updatedExisting" : false, "n" : 1, "connectionId" : 1, "err" : null, "ok" : 1 }

,
"value" : {

},
"ok" : 1
}
> db.test.find()

{ "_id" : 1, "name" : "Object 1" } { "_id" : 2, "name" : "Object 2" }

> db.runCommand({findAndModify:"test", "query" :

{ "name" : "asdf" }

, "update" :

{ "_id" : 3, "name" : "Object 3" }

, "upsert" : true, "new":true } )
{
"lastErrorObject" :

{ "updatedExisting" : false, "n" : 1, "connectionId" : 1, "err" : null, "ok" : 1 }

,
"errmsg" : "exception: assertion db/ops/../../util/net/../../db/../bson/bsonobjbuilder.h:127",
"code" : 0,
"ok" : 0
}
> db.test.find()

{ "_id" : 1, "name" : "Object 1" } { "_id" : 2, "name" : "Object 2" } { "_id" : 3, "name" : "Object 3" }

Adding a value (any value) to the _id key in the query document prevents an exception from being thrown.
(Example done with 2.0.1)
> db.runCommand({findAndModify:"test", "query" :

{ _id:0, "name" : "asdf" }

, "update" :

{ "_id" : 4, "name" : "Object 4" }

, "upsert" : true, "new":true } )
{
"lastErrorObject" :

{ "updatedExisting" : false, "n" : 1, "connectionId" : 1, "err" : null, "ok" : 1 }

,
"value" : {

},
"ok" : 1
}
> db.runCommand({findAndModify:"test", "query" :

{ _id:5, "name" : "asdf" }

, "update" :

{ "_id" : 5, "name" : "Object 5" }

, "upsert" : true, "new":true } )
{
"lastErrorObject" :

{ "updatedExisting" : false, "n" : 1, "connectionId" : 1, "err" : null, "ok" : 1 }

,
"value" :

{ "_id" : 5, "name" : "Object 5" }

,
"ok" : 1
}
> db.runCommand({findAndModify:"test", "query" :

{ _id:5, "name" : "asdf" }

, "update" :

{ "_id" : 6, "name" : "Object 6" }

, "upsert" : true, "new":true } )
{
"lastErrorObject" :

{ "updatedExisting" : false, "n" : 1, "connectionId" : 1, "err" : null, "ok" : 1 }

,
"value" :

{ "_id" : 5, "name" : "Object 5" }

,
"ok" : 1
}