After running JS to atomically update a number of documents, document data was moved and JSON elements re-ordered. I think this is expected. When this happened, the _class element appeared before the _id element. This seemed to impacting the ability to update existing records. Updates were silently failing with no update to the document.

Here's a snippet of the JSON:

{
   "_class": "com.riskchecknow.datafactory.model.person.impl.PersonWatchImpl",
   "_id": "ObjectId("50254761e4b01356f16783f9")",
   "active": true,
   "createdAt": "ISODate("2012-08-10T17: 39: 45.226Z")",
   "shardk": {
     "value": "47onmsprlrvjrqtmbvgf29tuk8",
     "date": "ISODate("2012-08-01T04: 00: 00.0Z")" 
  },
   "updatedAt": "ISODate("2012-08-10T17: 39: 49.299Z")", ...

Work around was to rename the collection, read through the renamed collection writing everything back the original collection.

Is there a known bug that would prevent proper update behavior when _id is not the first element in the document?