How to get pointer of current document to update in updateMany

Question

I have a latest mongodb 3.2 and there is a collection of many items that have timeStamp.

A need to convert milliseconds to Date object and now I use this function:

db.myColl.find().forEach(function (doc) {
    doc.date = new Date(doc.date);
    db.myColl.save(doc);
})

It took very long time to update 2 millions of rows.

I try to use updateMany (seems it is very fast) but how I can get access to a current document? Is there any chance to rewrite the query above by using updateMany?

Thank you.

Answer 1

You can leverage other bulk update APIs like the bulkWrite() method which will allow you to use an iterator to access a document, manipulate it, add the modified document to a list and then send the list of the update operations in a batch to the server for execution.

The following demonstrates this approach, in which you would use the cursor's forEach() method to iterate the colloction and modify the each document at the same time pushing the update operation to a batch of about 1000 documents which can then be updated at once using the bulkWrite() method.

This is as efficient as using the updateMany() since it uses the same underlying bulk write operations:

var cursor = db.myColl.find({"date": { "$exists": true, "$type": 1 }}),
    bulkUpdateOps = [];

cursor.forEach(function(doc){ 
    var newDate = new Date(doc.date);
    bulkUpdateOps.push({ 
        "updateOne": {
            "filter": { "_id": doc._id },
            "update": { "$set": { "date": newDate } }
         }
    });

    if (bulkUpdateOps.length == 1000) {
        db.myColl.bulkWrite(bulkUpdateOps);
        bulkUpdateOps = [];
    }
});         

if (bulkUpdateOps.length > 0) { db.myColl.bulkWrite(bulkUpdateOps); }