> db.foo.insert(

{"items":[null,null,null,null]}

)
> db.foo.find()

{ "_id" : ObjectId("4bac322669b89c9179dc9976"), "items" : [ null, null, null, null ] }

> db.foo.update({_id:ObjectId("4bac322669b89c9179dc9976")},{$set:{"items.0":

{itemId:1,amount:9000}

}})
> db.foo.find()
{ "_id" : ObjectId("4bac322669b89c9179dc9976"), "items" :
[

{ "itemId" : 1, "amount" : 9000 }

, null, null, null ] }
> db.foo.update({_id:ObjectId("4bac322669b89c9179dc9976")},{$inc:

{"items.0.amount":1000}

,$set:{"items.1":

{itemId:1,amount:1}

}})
> db.foo.find()
{ "_id" : ObjectId("4bac322669b89c9179dc9976"), "items" : [

{ "itemId" : 1, "amount" : 9000 }

,

{ "itemId" : 1, "amount" : 1 }

,

{ "amount" : 1000 }

,
null,
null
] }
>

I hope that the result should be
{ "_id" : ObjectId("4bac322669b89c9179dc9976"), "items" : [

{ "itemId" : 1, "amount" : 10000 }

,

{ "itemId" : 1, "amount" : 1 }

,
null,
null,
null
] }