Converting older Mongo database references to DBRefs - mongodb

Question

I'm in the process of updating some legacy software that is still running on Mongo 2.4. The first step is to upgrade to the latest 2.6 then go from there.
Running the db.upgradeCheckAllDBs(); gives us the DollarPrefixedFieldName: $id is not valid for storage. errors and indeed we have some older records with legacy $id, $ref fields. We have a number of collections that look something like this:
{
"_id" : "1",
"someRef" : {"$id" : "42", "$ref" : "someRef"}
},
{
"_id" : "2",
"someRef" : DBRef("someRef", "42")
},
{
"_id" : "3",
"someRef" : DBRef("someRef", "42")
},
{
"_id" : "4",
"someRef" : {"$id" : "42", "$ref" : "someRef"}
}
I want to script this to convert the older {"$id" : "42", "$ref" : "someRef"} objects to DBRef("someRef", "42") objects but leave the existing DBRef objects untouched. Unfortunately, I haven't been able to differentiate between the two types of objects.
Using typeof and $type simply say they are objects.
Both have $id and $ref fields.
In our groovy console when you pull one of the old ones back and one of the new ones getClass() returns DBRef for both.
We have about 80k records with this legacy format out of millions of total records. I'd hate to have to brute force it and modify every record whether it needs it or not.
This script will do what I need it to do but the find() will basically return all the records in the collection.
var cursor = db.someCollection.find({"someRef.$id" : {$exists: true}});
while(cursor.hasNext()) {
var rec = cursor.next();
db.someCollection.update({"_id": rec._id}, {$set: {"someRef": DBRef(rec.someRef.$ref, rec.someRef.$id)}});
}
Is there another way that I am missing that can be used to find only the offending records?
Update
As described in the accepted answer the order matters which made all the difference. The script we went with that corrected our data:
var cursor = db.someCollection.find(
{
$where: "function() { return this.someRef != null &&
Object.keys(this.someRef)[0] == '$id'; }"
}
);
while(cursor.hasNext()) {
var rec = cursor.next();
db.someCollection.update(
{"_id": rec._id},
{$set: {"someRef": DBRef(rec.someRef.$ref, rec.someRef.$id)}}
);
}
We did have a collection with a larger number of records that needed to be corrected where the connection timed out. We just ran the script again and it got through the remaining records.
There's probably a better way to do this. I would be interested in hearing about a better approach. For now, this problem is solved.

Answer 1

DBRef is a client side thing. http://docs.mongodb.org/manual/reference/database-references/#dbrefs says it pretty clear:
The order of fields in the DBRef matters, and you must use the above sequence when using a DBRef.
The drivers benefit from the fact that order of fields in BSON is consistent to recognise DBRef, so you can do the same:
db.someCollection.find({ $expr: {
$let: {
vars: {firstKey: { $arrayElemAt: [ { $objectToArray: "$someRef" }, 0] } },
in: { $eq: [{ $substr: [ "$$firstKey.k", 1, 2 ] } , "id"]}
}
} } )
will return objects where order of the fields doesn't match driver's expectation.