Mongodb impossible () E11000 duplicate key error dup key when upserting

我的理解是，在单个文档上使用upsert:true进行更新是一个原子操作，因此当集合没有唯一的索引字段时，这不会导致重复键错误，尤其是在主_id键上:

Order.update({ _id: order._id }, query, { upsert: true }, cb) // with mongoose

但这出现在mongod上.日志(log):

    2015-03-27T09:39:10.349-0400 I WRITE    [conn258236] update xyz.orders 
query: { _id: "6353f880-c6a7-4260-809f-98e0af27b9a2" } update: { $set: { ... 
} keyUpdates:0 writeConflicts:0 **exception: E11000 duplicate key error dup 
key: { : "6353f880-c6a7-4260-809f-98e0af27b9a2" } code:11000** numYields:1 
locks:{} 138ms


    2015-03-27T09:39:10.349-0400 I COMMAND  [conn258236] command xyz.$cmd 
command: update { update: "orders", writeConcern: { w: 1 }, ordered: true, 
updates: [ { q: { _id: "6353f880-c6a7-4260-809f-98e0af27b9a2" }, u: { $set: { 
... } }, multi: false, upsert: true } ] } keyUpdates:0 writeConflicts:0 
numYields:0 reslen:235 locks:{} 139ms

以下是db.orders.getIndexes()的输出:

{
    "v" : 1,
    "key" : {
        "_id" : 1
    },
    "name" : "_id_",
    "ns" : "xyz.orders"
},

我们正在使用MongoDB 3.0.0版和WiredTiger.