Nrthugu

You should iterate through. For your specific case:

db.person.find().snapshot().forEach(

    function (elem) {

        db.person.update(

            {

                _id: elem._id

            },

            {

                $set: {

                    name: elem.firstname + ' ' + elem.lastname

                }

            }

        );

    }

);

The best way to do this is to use the aggregation framework to compute our new field.

MongoDB 3.4

The most efficient solution is in MongoDB 3.4 using the $addFields and the $out aggregation pipeline operators.

db.collection.aggregate(

    [

        { "$addFields": { 

            "name": { "$concat": [ "$firstName", " ", "$lastName" ] } 

        }},

        { "$out": "collection" }

    ]

)

Note that this does not update your collection but instead replace the existing collection or create a new one. Also for update operations that require "type casting" you will need client side processing, and depending on the operation, you may need to use the find() method instead of the .aggreate() method.

MongoDB 3.2 and 3.0

The way we do this is by $projecting our documents and use the $concat string aggregation operator to return the concatenated string.
we From there, you then iterate the cursor and use the $set update operator to add the new field to your documents using bulk operations for maximum efficiency.

Aggregation query:

var cursor = db.collection.aggregate([ 

    { "$project":  { 

        "name": { "$concat": [ "$firstName", " ", "$lastName" ] } 

    }}

])

MongoDB 3.2 or newer

from this, you need to use the bulkWrite method.

var requests = ;

cursor.forEach(document => { 

    requests.push( { 

        'updateOne': {

            'filter': { '_id': document._id },

            'update': { '$set': { 'name': document.name } }

        }

    });

    if (requests.length === 500) {

        //Execute per 500 operations and re-init

        db.collection.bulkWrite(requests);

        requests = ;

    }

});



if(requests.length > 0) {

     db.collection.bulkWrite(requests);

}

MongoDB 2.6 and 3.0

From this version you need to use the now deprecated Bulk API and its associated methods.

var bulk = db.collection.initializeUnorderedBulkOp();

var count = 0;



cursor.snapshot().forEach(function(document) { 

    bulk.find({ '_id': document._id }).updateOne( {

        '$set': { 'name': document.name }

    });

    count++;

    if(count%500 === 0) {

        // Excecute per 500 operations and re-init

        bulk.execute();

        bulk = db.collection.initializeUnorderedBulkOp();

    }

})



// clean up queues

if(count > 0) {

    bulk.execute();

}

MongoDB 2.4

cursor["result"].forEach(function(document) {

    db.collection.update(

        { "_id": document._id }, 

        { "$set": { "name": document.name } }

    );

})

For a database with high activity, you may run into issues where your updates affect actively changing records and for this reason I recommend using snapshot()

db.person.find().snapshot().forEach( function (hombre) {

    hombre.name = hombre.firstName + ' ' + hombre.lastName; 

    db.person.save(hombre); 

});

http://docs.mongodb.org/manual/reference/method/cursor.snapshot/

I tried the above solution but I found it unsuitable for large amounts of data. I then discovered the stream feature:

MongoClient.connect("...", function(err, db){

    var c = db.collection('yourCollection');

    var s = c.find({/* your query */}).stream();

    s.on('data', function(doc){

        c.update({_id: doc._id}, {$set: {name : doc.firstName + ' ' + doc.lastName}}, function(err, result) { /* result == true? */} }

    });

    s.on('end', function(){

        // stream can end before all your updates do if you have a lot

    })

})

Here's what we came up with for copying one field to another for ~150_000 records. It took about 6 minutes, but is still significantly less resource intensive than it would have been to instantiate and iterate over the same number of ruby objects.

js_query = %({

  $or : [

    {

      'settings.mobile_notifications' : { $exists : false },

      'settings.mobile_admin_notifications' : { $exists : false }

    }

  ]

})



js_for_each = %(function(user) {

  if (!user.settings.hasOwnProperty('mobile_notifications')) {

    user.settings.mobile_notifications = user.settings.email_notifications;

  }

  if (!user.settings.hasOwnProperty('mobile_admin_notifications')) {

    user.settings.mobile_admin_notifications = user.settings.email_admin_notifications;

  }

  db.users.save(user);

})



js = "db.users.find(#{js_query}).forEach(#{js_for_each});"

Mongoid::Sessions.default.command('$eval' => js)