Insert field with array size in mongo

Question

I have a documents in mongodb, containing some array. Now I need to have a field containing a quantity of items of this array. So I need to update documents adding this fiel

Answer 1

You could initialise a Bulk() operations builder to update the document in a loop as follows:

var bulk = db.collection.initializeOrderedBulkOp(),   
    count = 0;

db.collection.find("itemsTotal": { "$exists": false },
     "items": {
         $exists: true
     }
).forEach(function(doc) { 
    var items_size = doc.items.length;
    bulk.find({ "_id": doc._id }).updateOne({ 
        "$set": { "itemsTotal": items_size }
    });
    count++;
    if (count % 100 == 0) {
        bulk.execute();
        bulk = db.collection.initializeUnorderedBulkOp();
    }
});

if (count % 100 != 0) { bulk.execute(); }