Cfrgtkky

In MongoDB, is it possible to update the value of a field using the value from another field? The equivalent SQL would be something like:

UPDATE Person SET Name = FirstName + ' ' + LastName

And the MongoDB pseudo-code would be:

db.person.update( {}, { $set : { name : firstName + ' ' + 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 } }

    );

})

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

                }

            }

        );

    }

);

Apparently there is a way to do this efficiently since MongoDB 3.4, see styvane's answer.

Obsolete answer below

You cannot refer to the document itself in an update (yet). You'll need to iterate through the documents and update each document using a function. See this answer for an example, or this one for server-side eval().