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 can use the .aggregate() method to $project your documents and return the $size of the items array. After that you will need to loop through your aggregation result using the .forEach loop and $set the itemTotal field for your document using "Bulk" operation for maximum efficiency.

var bulkOp = db.myDocument.initializeUnorderedBulkOp(); 
var count = 0;

db.myDocument.aggregate([
    { "$match": { 
        "itemsTotal": { "$exists": false } ,
        "items": { "$exists": true }
    }}, 
    { "$project": { "itemsTotal": { "$size": "$items" } } }
]).forEach(function(doc) { 
        bulkOp.find({ "_id": doc._id }).updateOne({ 
            "$set": { "itemsTotal": doc.itemsTotal }
        });
        count++;
        if (count % 200 === 0) {
            // Execute per 200 operations and re-init
            bulkOp.execute();
            bulkOp = db.myDocument.initializeUnorderedBulkOp();
        }
})

// Clean up queues
if (count > 0) { 
    bulkOp.execute();
}