MongoDB tip, identify and convert type of a specific field

I found this post when I encountered some type issue and the better solution seems to be to convert the type in place.

So I just did a quick experiment myself to make sure it works properly.

to set up this experiment, first run

db.testConvert.insert({typeField:NumberLong(1)});
db.testConvert.insert({typeField:NumberInt(1)});
db.testConvert.insert({typeField:1});

We verify now typeField has contain all three types, Int64, Int32, and Double

Referencing from BSON type documentation, we know that Double is 1.

Then we run the following

var bulk = db.testConvert.initializeUnorderedBulkOp(),
count=0;
db.testConvert.find({"typeField":{"$type":1}}).forEach(function(doc) {
    bulk.find({"_id":doc._id}).updateOne({
        "$set":{"tmpField":NumberLong(doc.typeField.valueOf())},
        "$unset":{"typeField":1}
    });
    bulk.find({"_id": doc._id}).updateOne({ "$rename": { "tmpField": "typeField" } });
    count++;
    if ( count % 1000 == 0 ) {
        bulk.execute()
        bulk = db.sample.initializeUnOrderedBulkOp();
    }
})
if ( count % 1000 != 0 ) bulk.execute();

Note:
"typeField":{"$type":1} => to look for the type "double"

 "$set":{"tmpField":NumberLong(doc.typeField.valueOf()) => get the value and convert to NumberLong to a temporary field

 "$unset":{"typeField":1} => delete "typeField"


Now we can see the script has picked up the entry with type double and convert to NumberLong() as we had specified.