关于mongo批量使用ObjectId替换新增字段的值遇到问题

数据库数据如下：

> db.food.find()
{ "_id" : ObjectId("551e02da874e367bbf0b218e"), "fruit" : [ "apple", "orange", "pear" ] }
{ "_id" : ObjectId("551e044e874e367bbf0b218f"), "fruit" : [ "strawberry", "orange" ] }
{ "_id" : ObjectId("551e01ca874e367bbf0b218b"), "fruit" : [ "apple", "banana", "peach", "apricot" ] }
{ "_id" : ObjectId("551e0220874e367bbf0b218c"), "fruit" : [ "apple", "orange", "cherry" ] }
{ "_id" : ObjectId("551e026a874e367bbf0b218d"), "fruit" : [ "strawberry", "orange", "pear" ] }

现在需求是新增字段keycode，使其值是ObjectId的值，使用如下Update语句，可以执行批量更新：

> db.food.find().forEach(
...    function(item){                 
...        db.food.update({"keycode":{$exists:false}},{"$set":{"keycode":item._id.valueOf()}}) 
...     }
... )
> db.food.find()
{ "_id" : ObjectId("551e02da874e367bbf0b218e"), "fruit" : [ "apple", "orange", "pear" ], "keycode" : "551e02da874e367bbf0b218e" }
{ "_id" : ObjectId("551e044e874e367bbf0b218f"), "fruit" : [ "strawberry", "orange" ], "keycode" : "551e044e874e367bbf0b218f" }
{ "_id" : ObjectId("551e01ca874e367bbf0b218b"), "fruit" : [ "apple", "banana", "peach", "apricot" ], "keycode" : "551e01ca874e367bbf0b218b" }
{ "_id" : ObjectId("551e0220874e367bbf0b218c"), "fruit" : [ "apple", "orange", "cherry" ], "keycode" : "551e0220874e367bbf0b218c" }
{ "_id" : ObjectId("551e026a874e367bbf0b218d"), "fruit" : [ "strawberry", "orange", "pear" ], "keycode" : "551e026a874e367bbf0b218d" }
现在遇到的问题是如果，我只更新符合条件的数据，却发现更新错误，如下：

> db.food.find()
{ "_id" : ObjectId("551e02da874e367bbf0b218e"), "fruit" : [ "apple", "orange", "pear" ], "keycode" : "551e02da874e367bbf0b218e" }
{ "_id" : ObjectId("551e044e874e367bbf0b218f"), "fruit" : [ "strawberry", "orange" ] }
{ "_id" : ObjectId("551e01ca874e367bbf0b218b"), "fruit" : [ "apple", "banana", "peach", "apricot" ], "keycode" : "551e01ca874e367bbf0b218b" }
{ "_id" : ObjectId("551e0220874e367bbf0b218c"), "fruit" : [ "apple", "orange", "cherry" ] }
{ "_id" : ObjectId("551e026a874e367bbf0b218d"), "fruit" : [ "strawberry", "orange", "pear" ] }
> db.food.find().forEach(
...    function(item){                 
...        db.food.update({"keycode":{$exists:false}},{"$set":{"keycode":item._id.valueOf()}}) 
...     }
... )
> db.food.find()
{ "_id" : ObjectId("551e02da874e367bbf0b218e"), "fruit" : [ "apple", "orange", "pear" ], "keycode" : "551e02da874e367bbf0b218e" }
{ "_id" : ObjectId("551e044e874e367bbf0b218f"), "fruit" : [ "strawberry", "orange" ], "keycode" : "551e02da874e367bbf0b218e" }
{ "_id" : ObjectId("551e01ca874e367bbf0b218b"), "fruit" : [ "apple", "banana", "peach", "apricot" ], "keycode" : "551e01ca874e367bbf0b218b" }
{ "_id" : ObjectId("551e0220874e367bbf0b218c"), "fruit" : [ "apple", "orange", "cherry" ], "keycode" : "551e044e874e367bbf0b218f" }
{ "_id" : ObjectId("551e026a874e367bbf0b218d"), "fruit" : [ "strawberry", "orange", "pear" ], "keycode" : "551e01ca874e367bbf0b218b" }

结果与预期不否，更新出错，不太明白为什么？