MongoDB concatenate strings from two fields into a third field

Question

How do I concatenate values from two fields and put it into a third one, the values are strings. I've tried this:

db.collection.update({"_id" : { $exists : true }},
                     {$set: {column_2:{$add:['$column_4',
                                             '$column_3']}}},
                     false, true)

doesn't seem to work though, throws not ok for storage. I've also tried this:

db.collection.update({"_id" : { $exists : true }},
                     {$set: {column_2:{$add:['a',
                                             'b']}}},
                     false, true)

but even this shows the same error not ok for storage.

I want to concatenate only on the mongo server and not in my application.

Answer 1

Unfortunately, MongoDB currently does not allow you to reference the existing value of any field when performing an update(). There is an existing Jira ticket to add this functionality: see SERVER-1765 for details.

At present, you must do an initial query in order to determine the existing values, and do the string manipulation in the client. I wish I had a better answer for you.

Answer 2

You can use aggregate, $project and $concat : https://docs.mongodb.org/v3.0/reference/operator/aggregation/project/ https://docs.mongodb.org/manual/reference/operator/aggregation/concat/

It would be something like this :

db.collection.aggregate(
   [
      { $project: { newfield: { $concat: [ "$field1", " - ", "$field2" ] } } }
   ]
)

Answer 3

let suppose that you have a collection name is "myData" where you have data like this

{
"_id":"xvradt5gtg",
"first_name":"nizam",
"last_name":"khan",
"address":"H-148, Near Hero Show Room, Shahjahanpur",
}

and you want concatenate fields (first_name+ last_name +address) and save it into "address" field like this

{
"_id":"xvradt5gtg",
"first_name":"nizam",
"last_name":"khan",
"address":"nizam khan,H-148, Near Hero Show Room, Shahjahanpur",
}

now write query will be

{
var x=db.myData.find({_id:"xvradt5gtg"});
x.forEach(function(d)
    { 
        var first_name=d.first_name;
        var last_name=d.last_name;
        var _add=d.address;  
        var fullAddress=first_name+","+last_name+","+_add; 
        //you can print also
        print(fullAddress); 
        //update 
        db.myData.update({_id:d._id},{$set:{address:fullAddress}});
    })
}

Answer 4

You can also follow the below.

db.collectionName.find({}).forEach(function(row) { 
    row.newField = row.field1 +"-" +row.field2
    db.collection.save(row);
});

Answer 5

db.myDB.find().forEach(function(e){db.myDB.update({"_id":e._id},{$set{"name":'More' + e.name + ' '}});

This is a solution!!