MongoDB update only a few records, create a new attribute, assign a value - mongodb

Question

I wasn't lucky finding any help on this anywhere else.
Basically I want to update only the few documents that do not have a given attribute already.
And the value for the update comes from a field that is already on the document.
This is what I tried but it didn't like the "from a field already on the document" part. Saying Cn doesn't exist.
db.getCollection('test').update(
// query
{ "id2" : { $exists: false } },
// update
{ id2: Cn },
// options
{
"multi" : true, // update all documents
"upsert" : false // don't insert new documents
}
);
Here is my test data
/* 1 */
{
"_id" : ObjectId("5912132c4a58677726d37168"),
"Cn" : "CA",
"id2" : "CAAB",
"Prov" : "AB"
}
/* 2 */
{
"_id" : ObjectId("591213404a58677726d37172"),
"Cn" : "CA",
"id2" : "CANZ",
"Prov" : "NZ"
}
/* 3 */
{
"_id" : ObjectId("591213534a58677726d37180"),
"Cn" : "CA",
"id2" : "CAMB",
"Prov" : "MB"
}
/* 4 */
{
"_id" : ObjectId("591213674a58677726d3718c"),
"Cn" : "US"
}
/* 5 */
{
"_id" : ObjectId("591213894a58677726d371a3"),
"Cn" : "MX"
}
All this should do is create a id2 on US & MX and give those new id2 attributes the corresponding values 'US' & 'MX'.
This would not be a big deal but I have able 144 countries & 10,000+ documents to add id2 to.

Answer 1

You can't update multiple items with an operator that only contains expressions, and self referencing doesn't work either.
Check out this question for work around solutions to get what you need done: Update MongoDB field using value of another field

Answer 2

You can try this :
db.your_collection.aggregate( [
{ $match : { id2 : { $exists: false }}},
{ $addFields: {
id2 : "$Cn"
}
},
{ $out : "your_collection" }
]);
This will remove all documents in collection which have id2 field.
You can do this in mongoose in a longer way :
db.getCollection('test').find({ "id2" : { $exists: false } }, function(err, docs){
if(err){
//handle errror
}
else if(!docs){
// no docs found
}
else{
for(var i=0; i< docs.length; i++){
db.getCollection('test')
.findByIdAndUpdate(
docs[i]._id
, { $set : { id2 : docs[i].Cn}}
, { new : true}
, function(err, doc){
if(err){
//handle error
}
else{
//doc was updated
}
});
}
}
});

Answer 3

I would not have gotten to this without Mihir Bhende pushing. Thanks for that.
var c = db.getCollection('test').find(
{ "id2" : { $exists: false }}
);
c.forEach(function(myDoc) {
print("doc", myDoc.Cn);
db.getCollection('test').update( {_id: myDoc._id}, {$set: { "id2": myDoc.Cn }}, function (err) {
if (err) { print("err"); }
});
});