MongoDB concatenate strings from two fields into a third field

Question

How do I concatenate values from two string fields and put it into a third one?

I've tried this:

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

which doesn't seem to work though, and 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.

Swanhilda

see <https://mcmap.net/q/500151/-mongodb-concatenate-results>? – Malfunction

forEach is inefficient, multiple update is what I'm looking for. Also it doesn't explain, how to update the value of another field with the concatenated string, this is what I'm looking for col3 = col1 + col2 "+" implying concatenation – Swanhilda

Mongodb does not, atm, allow the relfection of it's own document fields within the query without using JS functions. I would strongly advise not using the JS functions since they have been proven to make a query 10x slower and also have other problems. You will need to find some creative way of solving this – Alcantar

As to normal string concatenation you dont need a $add op since it is isn't like SQL, instead you can just do column_2:'a'+'b' – Alcantar

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.

Tutelary

FYI (7 years later) MongoDB has multiple ways to accomplish this without client-side code. This was the best answer in 2012, but William Z's wish for a better answer has been granted. :) – Umbrage

Answer 2

You can use aggregation operators $project and $concat:

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

Genetic

The correct answer would include $out as the last pipeline stage, to output in the same collection (or a new one), since OP wants to update the collection – Salesclerk

MongoDB 4.2 now has a $merge pipeline stage which has some advantages over $out, depending on your performance, concurrency and consistency requirements. – Umbrage

What would the above aggregation look like with a step that updates the existing document with the new concatenated field? – Alary

Answer 3

You could use $set like this in 4.2 which supports aggregation pipeline in update.

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

Ekaterinoslav

Answer 4

Building on the answer from @rebe100x, as suggested by @Jamby ...

You can use $project, $concat and $out (or $merge) in an aggregation pipeline. https://docs.mongodb.org/v3.0/reference/operator/aggregation/project/ https://docs.mongodb.org/manual/reference/operator/aggregation/concat/ https://docs.mongodb.com/manual/reference/operator/aggregation/out/

For example:

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

With MongoDB 4.2 . . .

MongoDB 4.2 adds the $merge pipeline stage which offers selective replacement of documents within the collection, while $out would replace the entire collection. You also have the option of merging instead of replacing the target document.

    db.collection.aggregate(
       [
          { $project: { newfield: { $concat: [ "$field1", " - ", "$field2" ] } } },
          { $merge: { into: "collection", on: "_id", whenMatched: "merge", whenNotMatched: "discard" }
       ]
    )

You should consider the trade-offs between performance, concurrency and consistency, when choosing between $merge and $out, since $out will atomically perform the collection replacement via a temporary collection and renaming.

https://docs.mongodb.com/manual/reference/operator/aggregation/merge/ https://docs.mongodb.com/manual/reference/operator/aggregation/merge/#merge-out-comparison

Umbrage

Answer 5

**

in my case this $concat worked for me ...

**

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

Numerate

Answer 6

You can also follow the below.

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

Freewill

Answer 7

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}});
    })
}

Oversell

Answer 8

Find and Update Each Using For Loop

1. Try This:

   db.getCollection('users').find({ }).forEach( function(user) {
       user.full_name = user.first_name + " " + user.last_name;
       db.getCollection('users').save(user);
   });
   

2. Or Try This:

   db.getCollection('users').find({ }).forEach( function(user) {
       db.getCollection('users').update(
           { _id: user._id },
           { $set: { "full_name": user.first_name + " " + user.last_name } }
       )
   });
   

Epidaurus